203 lines
8.7 KiB
Python
203 lines
8.7 KiB
Python
# Inspired by https://github.com/facebookresearch/xformers/blob/main/xformers/components/positional_embedding/rotary.py
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from typing import Tuple
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import math
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import torch
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from einops import rearrange, repeat
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import rotary_emb
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def rotate_half(x):
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x1, x2 = x.chunk(2, dim=-1)
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return torch.cat((-x2, x1), dim=-1)
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def apply_rotary_emb_torch(x, cos, sin):
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"""
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x: (batch_size, seqlen, nheads, headdim)
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cos, sin: (seqlen, rotary_dim / 2)
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"""
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rotary_dim = cos.shape[-1] * 2
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assert rotary_dim <= x.shape[-1]
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cos = repeat(cos, 's d -> s 1 (2 d)')
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sin = repeat(sin, 's d -> s 1 (2 d)')
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return torch.cat([x[..., :rotary_dim] * cos + rotate_half(x[..., :rotary_dim]) * sin,
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x[..., rotary_dim:]], dim=-1)
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class ApplyRotaryEmb(torch.autograd.Function):
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@staticmethod
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def forward(ctx, x, cos, sin, inplace=False):
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"""
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x: (batch_size, seqlen, nheads, headdim)
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cos, sin: (seqlen, rotary_dim / 2)
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rotary_dim must be <= headdim
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Apply rotary embedding to the first rotary_dim of x.
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"""
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batch, seqlen, nheads, headdim = x.shape
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rotary_seqlen, rotary_dim = cos.shape
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rotary_dim *= 2
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assert rotary_dim <= headdim
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assert seqlen <= rotary_seqlen
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assert sin.shape == (rotary_seqlen, rotary_dim // 2)
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x1, x2 = x[..., :rotary_dim].chunk(2, dim=-1)
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out = torch.empty_like(x) if not inplace else x
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o1, o2 = out[..., :rotary_dim].chunk(2, dim=-1) if not inplace else (x1, x2)
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rotary_emb.apply_rotary(x1, x2, rearrange(cos[:seqlen], 's d -> s 1 d'),
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rearrange(sin[:seqlen], 's d -> s 1 d'), o1, o2, False)
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if not inplace and rotary_dim < headdim:
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out[..., rotary_dim:].copy_(x[..., rotary_dim:])
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ctx.save_for_backward(cos, sin)
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ctx.inplace = inplace
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return out if not inplace else x
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@staticmethod
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def backward(ctx, do):
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cos, sin = ctx.saved_tensors
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_, seqlen, _, headdim = do.shape
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rotary_dim = cos.shape[-1]
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rotary_dim *= 2
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inplace = ctx.inplace
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do1, do2 = do[..., :rotary_dim].chunk(2, dim=-1)
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dx = torch.empty_like(do) if not inplace else do
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dx1, dx2 = dx[..., :rotary_dim].chunk(2, dim=-1) if not inplace else (do1, do2)
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rotary_emb.apply_rotary(do1, do2, rearrange(cos[:seqlen], 's d -> s 1 d'),
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rearrange(sin[:seqlen], 's d -> s 1 d'), dx1, dx2, True)
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if not inplace and rotary_dim < headdim:
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dx[..., rotary_dim:].copy_(do[..., rotary_dim:])
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return dx, None, None, None
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apply_rotary_emb_func = ApplyRotaryEmb.apply
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class ApplyRotaryEmbQKV_(torch.autograd.Function):
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@staticmethod
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def forward(ctx, qkv, cos, sin, cos_k=None, sin_k=None):
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"""
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qkv: (batch_size, seqlen, 3, nheads, headdim)
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cos, sin: (seqlen, rotary_dim / 2)
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cos_k, sin_k: (seqlen, rotary_dim / 2), optional
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rotary_dim must be <= headdim
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Apply rotary embedding *inplace* to the first rotary_dim of q and k.
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"""
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batch, seqlen, three, nheads, headdim = qkv.shape
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assert three == 3
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rotary_seqlen, rotary_dim = cos.shape
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rotary_dim *= 2
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assert rotary_dim <= headdim
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assert seqlen <= rotary_seqlen
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cos_k = cos if cos_k is None else cos_k
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sin_k = sin if sin_k is None else sin_k
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assert sin.shape == cos_k.shape == sin_k.shape == (rotary_seqlen, rotary_dim // 2)
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q1, q2 = qkv[:, :, 0, :, :rotary_dim].chunk(2, dim=-1)
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rotary_emb.apply_rotary(q1, q2, rearrange(cos[:seqlen], 's d -> s 1 d'),
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rearrange(sin[:seqlen], 's d -> s 1 d'), q1, q2, False)
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k1, k2 = qkv[:, :, 1, :, :rotary_dim].chunk(2, dim=-1)
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rotary_emb.apply_rotary(k1, k2, rearrange(cos_k[:seqlen], 's d -> s 1 d'),
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rearrange(sin_k[:seqlen], 's d -> s 1 d'), k1, k2, False)
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ctx.save_for_backward(cos, sin, cos_k, sin_k)
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return qkv
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@staticmethod
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def backward(ctx, dqkv):
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cos, sin, cos_k, sin_k = ctx.saved_tensors
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_, seqlen, _, _, headdim = dqkv.shape
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rotary_dim = cos.shape[-1]
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rotary_dim *= 2
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dq1, dq2 = dqkv[:, :, 0, :, :rotary_dim].chunk(2, dim=-1)
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rotary_emb.apply_rotary(dq1, dq2, rearrange(cos[:seqlen], 's d -> s 1 d'),
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rearrange(sin[:seqlen], 's d -> s 1 d'), dq1, dq2, True)
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dk1, dk2 = dqkv[:, :, 1, :, :rotary_dim].chunk(2, dim=-1)
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rotary_emb.apply_rotary(dk1, dk2, rearrange(cos_k[:seqlen], 's d -> s 1 d'),
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rearrange(sin_k[:seqlen], 's d -> s 1 d'), dk1, dk2, True)
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return dqkv, None, None, None, None
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apply_rotary_emb_qkv_ = ApplyRotaryEmbQKV_.apply
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class RotaryEmbedding(torch.nn.Module):
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"""
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The rotary position embeddings from RoFormer_ (Su et. al).
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A crucial insight from the method is that the query and keys are
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transformed by rotation matrices which depend on the relative positions.
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Other implementations are available in the Rotary Transformer repo_ and in
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GPT-NeoX_, GPT-NeoX was an inspiration
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.. _RoFormer: https://arxiv.org/abs/2104.09864
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.. _repo: https://github.com/ZhuiyiTechnology/roformer
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.. _GPT-NeoX: https://github.com/EleutherAI/gpt-neox
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If scale_base > 0, this implements XPos (Sun et al., https://arxiv.org/abs/2212.10554).
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A recommended value for scale_base is 512: https://github.com/HazyResearch/flash-attention/issues/96
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Reference: https://github.com/sunyt32/torchscale/blob/main/torchscale/component/xpos_relative_position.py
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"""
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def __init__(self, dim: int, base=10000, scale_base=0, device=None):
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"""
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"""
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super().__init__()
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# Generate and save the inverse frequency buffer (non trainable)
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inv_freq = 1.0 / (base ** (torch.arange(0, dim, 2, device=device,
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dtype=torch.float32) / dim))
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self.register_buffer("inv_freq", inv_freq)
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self.scale_base = scale_base
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scale = ((torch.arange(0, dim, 2, device=device, dtype=torch.float32) + 0.4 * dim)
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/ (1.4 * dim) if scale_base > 0 else None)
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self.register_buffer("scale", scale)
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self._seq_len_cached = 0
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self._cos_cached = None
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self._sin_cached = None
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self._cos_k_cached = None
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self._sin_k_cached = None
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def _update_cos_sin_cache(self, x, seqlen_offset=0):
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"""x: (batch, seqlen, nheads, headdim) or (batch, seqlen, 3, nheads, headdim)
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"""
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seqlen = x.shape[1] + seqlen_offset
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# Reset the tables if the sequence length has changed,
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# or if we're on a new device (possibly due to tracing for instance)
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if (seqlen > self._seq_len_cached or self._cos_cached.device != x.device
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or self._cos_cached.dtype != x.dtype):
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self._seq_len_cached = seqlen
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t = torch.arange(seqlen, device=x.device, dtype=self.inv_freq.dtype)
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# Don't do einsum, it converts fp32 to fp16
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# freqs = torch.einsum("i,j->ij", t, self.inv_freq)
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freqs = torch.outer(t, self.inv_freq.to(device=t.device))
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if self.scale is None:
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self._cos_cached = torch.cos(freqs).to(x.dtype)
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self._sin_cached = torch.sin(freqs).to(x.dtype)
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else:
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power = ((torch.arange(seqlen, dtype=self.scale.dtype, device=self.scale.device)
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- seqlen // 2) / self.scale_base)
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scale = self.scale.to(device=power.device) ** rearrange(power, 's -> s 1')
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# We want the multiplication by scale to happen in fp32
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self._cos_cached = (torch.cos(freqs) * scale).to(x.dtype)
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self._sin_cached = (torch.sin(freqs) * scale).to(x.dtype)
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self._cos_k_cached = (torch.cos(freqs) / scale).to(x.dtype)
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self._sin_k_cached = (torch.sin(freqs) / scale).to(x.dtype)
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def forward(self, qkv: torch.Tensor, seqlen_offset: int = 0) -> Tuple[torch.Tensor, torch.Tensor]:
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"""
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seqlen_offset: can be used in generation where the qkv being passed in is only the last
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token in the batch.
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"""
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self._update_cos_sin_cache(qkv, seqlen_offset)
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if self.scale is None:
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return apply_rotary_emb_qkv_(
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qkv, self._cos_cached[seqlen_offset:], self._sin_cached[seqlen_offset:]
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)
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else:
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return apply_rotary_emb_qkv_(
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qkv, self._cos_cached[seqlen_offset:], self._sin_cached[seqlen_offset:],
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self._cos_k_cached[seqlen_offset:], self._sin_k_cached[seqlen_offset:]
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)
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