219 lines
9.7 KiB
Python
219 lines
9.7 KiB
Python
# Adapted from https://github.com/mlcommons/training_results_v1.1/blob/main/NVIDIA/benchmarks/bert/implementations/pytorch/padding.py
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import torch
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import torch.nn.functional as F
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from einops import rearrange, repeat
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class IndexFirstAxis(torch.autograd.Function):
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@staticmethod
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def forward(ctx, input, indices):
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ctx.save_for_backward(indices)
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assert input.ndim >= 2
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ctx.first_axis_dim, other_shape = input.shape[0], input.shape[1:]
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second_dim = other_shape.numel()
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# TD [2022-03-04] For some reason torch.gather is a bit faster than indexing.
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# return input[indices]
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return torch.gather(
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rearrange(input, "b ... -> b (...)"), 0, repeat(indices, "z -> z d", d=second_dim)
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).reshape(-1, *other_shape)
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@staticmethod
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def backward(ctx, grad_output):
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(indices,) = ctx.saved_tensors
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assert grad_output.ndim >= 2
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other_shape = grad_output.shape[1:]
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grad_output = rearrange(grad_output, "b ... -> b (...)")
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grad_input = torch.zeros(
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[ctx.first_axis_dim, grad_output.shape[1]],
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device=grad_output.device,
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dtype=grad_output.dtype,
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)
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# TD [2022-03-04] For some reason torch.scatter is a bit faster than indexing.
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# grad_input[indices] = grad_output
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grad_input.scatter_(0, repeat(indices, "z -> z d", d=grad_output.shape[1]), grad_output)
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return grad_input.reshape(ctx.first_axis_dim, *other_shape), None
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index_first_axis = IndexFirstAxis.apply
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class IndexPutFirstAxis(torch.autograd.Function):
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@staticmethod
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def forward(ctx, values, indices, first_axis_dim):
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ctx.save_for_backward(indices)
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assert indices.ndim == 1
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assert values.ndim >= 2
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output = torch.zeros(
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first_axis_dim, *values.shape[1:], device=values.device, dtype=values.dtype
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)
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# TD [2022-03-04] For some reason torch.scatter is a bit faster than indexing.
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output[indices] = values
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# output.scatter_(0, repeat(indices, 'z -> z d', d=values.shape[1]), values)
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return output
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@staticmethod
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def backward(ctx, grad_output):
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(indices,) = ctx.saved_tensors
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# TD [2022-03-04] For some reason torch.gather is a bit faster than indexing.
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grad_values = grad_output[indices]
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# grad_values = torch.gather(grad_output, 0, repeat(indices, 'z -> z d', d=grad_output.shape[1]))
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return grad_values, None, None
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index_put_first_axis = IndexPutFirstAxis.apply
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class IndexFirstAxisResidual(torch.autograd.Function):
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@staticmethod
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def forward(ctx, input, indices):
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ctx.save_for_backward(indices)
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assert input.ndim >= 2
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ctx.first_axis_dim, other_shape = input.shape[0], input.shape[1:]
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second_dim = other_shape.numel()
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# TD [2022-03-04] For some reason torch.gather is a bit faster than indexing.
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output = input[indices]
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# We don't want to reshape input (b ... -> b (...)) since it could change the channel_last
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# memory format to channel_first. In other words, input might not be contiguous.
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# If we don't detach, Pytorch complains about output being a view and is being modified inplace
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return output, input.detach()
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@staticmethod
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def backward(ctx, grad_output, grad_residual):
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(indices,) = ctx.saved_tensors
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assert grad_output.ndim >= 2
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other_shape = grad_output.shape[1:]
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assert grad_residual.shape[1:] == other_shape
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grad_input = grad_residual
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# grad_input[indices] += grad_output
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indices = indices.reshape(indices.shape[0], *((1,) * (grad_output.ndim - 1)))
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indices = indices.expand_as(grad_output)
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grad_input.scatter_add_(0, indices, grad_output)
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return grad_input.reshape(ctx.first_axis_dim, *other_shape), None
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index_first_axis_residual = IndexFirstAxisResidual.apply
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def unpad_input(hidden_states, attention_mask, unused_mask=None):
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"""
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Arguments:
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hidden_states: (batch, seqlen, ...)
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attention_mask: (batch, seqlen), bool / int, 1 means valid and 0 means not valid.
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unused_mask: (batch, seqlen), bool / int, 1 means the element is allocated but unused.
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Return:
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hidden_states: (total_nnz, ...), where total_nnz = number of tokens selected in attention_mask + unused_mask.
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indices: (total_nnz), the indices of masked tokens from the flattened input sequence.
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cu_seqlens: (batch + 1), the cumulative sequence lengths, used to index into hidden_states.
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max_seqlen_in_batch: int
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seqused: (batch), returns the number of tokens selected in attention_mask + unused_mask.
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"""
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all_masks = (attention_mask + unused_mask) if unused_mask is not None else attention_mask
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seqlens_in_batch = all_masks.sum(dim=-1, dtype=torch.int32)
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used_seqlens_in_batch = attention_mask.sum(dim=-1, dtype=torch.int32)
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indices = torch.nonzero(all_masks.flatten(), as_tuple=False).flatten()
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max_seqlen_in_batch = seqlens_in_batch.max().item()
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cu_seqlens = F.pad(torch.cumsum(seqlens_in_batch, dim=0, dtype=torch.torch.int32), (1, 0))
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# TD [2022-03-04] We don't want to index with a bool mask, because Pytorch will expand the
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# bool mask, then call nonzero to get the indices, then index with those. The indices is @dim
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# times larger than it needs to be, wasting memory. It's faster and more memory-efficient to
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# index with integer indices. Moreover, torch's index is a bit slower than it needs to be,
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# so we write custom forward and backward to make it a bit faster.
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return (
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index_first_axis(rearrange(hidden_states, "b s ... -> (b s) ..."), indices),
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indices,
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cu_seqlens,
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max_seqlen_in_batch,
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used_seqlens_in_batch,
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)
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def unpad_input_for_concatenated_sequences(hidden_states, attention_mask_in_length):
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"""
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Supports concatenating short samples in one sequence. The attention_mask_in_length is utilized to mask other short samples. It helps efficient training of variant lengths-based samples (e.g., the supervised fine-tuning task in large language model).
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The motivation for this function is explained [here](https://github.com/Dao-AILab/flash-attention/issues/432#issuecomment-1668822286).
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For example, if batch = 3 and seqlen = 6, the attention_mask_in_length is:
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```
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[
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[2, 3, 0, 0, 0, 0],
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[3, 2, 0, 0, 0, 0],
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[6, 0, 0, 0, 0, 0]
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]
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```
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, which refers to the 3D-attention mask:
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```
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[
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[
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[1, 0, 0, 0, 0, 0],
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[1, 1, 0, 0, 0, 0],
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[0, 0, 1, 0, 0, 0],
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[0, 0, 1, 1, 0, 0],
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[0, 0, 1, 1, 1, 0],
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[0, 0, 0, 0, 0, 1]
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],
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[
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[1, 0, 0, 0, 0, 0],
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[1, 1, 0, 0, 0, 0],
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[1, 1, 1, 0, 0, 0],
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[0, 0, 0, 1, 0, 0],
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[0, 0, 0, 1, 1, 0],
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[0, 0, 0, 0, 0, 1]
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],
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[
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[1, 0, 0, 0, 0, 0],
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[1, 1, 0, 0, 0, 0],
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[1, 1, 1, 0, 0, 0],
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[1, 1, 1, 1, 0, 0],
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[1, 1, 1, 1, 1, 0],
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[1, 1, 1, 1, 1, 1]
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]
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]
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```.
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Arguments:
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hidden_states: (batch, seqlen, ...)
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attention_mask_in_length: (batch, seqlen), int, a nonzero number (e.g., 1, 2, 3, etc.) means length of concatenated sequence in b-th batch, and 0 means none.
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Return:
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hidden_states: (total_nnz, ...), where total_nnz = number of tokens in selected in attention_mask.
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indices: (total_nnz), the indices of non-masked tokens from the flattened input sequence.
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cu_seqlens: (batch + 1), the cumulative sequence lengths, used to index into hidden_states.
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max_seqlen_in_batch: int
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"""
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length = attention_mask_in_length.sum(dim=-1)
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seqlen = attention_mask_in_length.size(-1)
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attention_mask_2d = torch.arange(seqlen, device=length.device, dtype=length.dtype).expand(len(length), seqlen) < length.unsqueeze(1)
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real_indices_idx = torch.nonzero(attention_mask_in_length.flatten(), as_tuple=False).flatten()
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seqlens_in_batch = attention_mask_in_length.flatten()[real_indices_idx]
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indices = torch.nonzero(attention_mask_2d.flatten(), as_tuple=False).flatten()
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max_seqlen_in_batch = seqlens_in_batch.max().item()
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cu_seqlens = F.pad(torch.cumsum(seqlens_in_batch, dim=0, dtype=torch.torch.int32), (1, 0))
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# TD [2022-03-04] We don't want to index with a bool mask, because Pytorch will expand the
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# bool mask, then call nonzero to get the indices, then index with those. The indices is @dim
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# times larger than it needs to be, wasting memory. It's faster and more memory-efficient to
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# index with integer indices. Moreover, torch's index is a bit slower than it needs to be,
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# so we write custom forward and backward to make it a bit faster.
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return (
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index_first_axis(rearrange(hidden_states, "b s ... -> (b s) ..."), indices),
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indices,
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cu_seqlens,
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max_seqlen_in_batch,
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)
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def pad_input(hidden_states, indices, batch, seqlen):
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"""
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Arguments:
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hidden_states: (total_nnz, ...), where total_nnz = number of tokens in selected in attention_mask.
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indices: (total_nnz), the indices that represent the non-masked tokens of the original padded input sequence.
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batch: int, batch size for the padded sequence.
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seqlen: int, maximum sequence length for the padded sequence.
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Return:
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hidden_states: (batch, seqlen, ...)
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"""
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dim = hidden_states.shape[-1]
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# output = torch.zeros((batch * seqlen), dim, device=hidden_states.device, dtype=hidden_states.dtype)
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# output[indices] = hidden_states
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output = index_put_first_axis(hidden_states, indices, batch * seqlen)
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return rearrange(output, "(b s) ... -> b s ...", b=batch)
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