[Bugfix] Add verbose error if scipy is missing for blocksparse attention (#5695)

vllm/attention/ops/blocksparse_attention/utils.py

@@ -6,7 +6,14 @@ from functools import lru_cache
 
 import torch
 import triton
+
+try:
     from scipy import sparse
+except ImportError as err:
+    raise ImportError("Please install scipy via "
+                      "`pip install scipy` to use "
+                      "BlockSparseAttention in "
+                      "models such as Phi-3.") from err
 
 
 def dense_to_crow_col(x: torch.Tensor):