[Deepseek V3.2] Use torch.compile to speed up torch.cat in nsa (#13022)

Signed-off-by: Hao Lu <14827759+hlu1@users.noreply.github.com>

python/sglang/srt/layers/attention/nsa_backend.py

@@ -123,6 +123,27 @@ class TopkTransformMethod(IntEnum):
     RAGGED = auto()
 
 
+@torch.compile
+def _compiled_cat(tensors: list[torch.Tensor], dim: int = -1) -> torch.Tensor:
+    return torch.cat(tensors, dim=dim)
+
+
+def _cat(tensors: list[torch.Tensor], dim: int = -1) -> torch.Tensor:
+    """
+    Concatenate two tensors along the last dimension.
+    Use this function to concatenate q_nope and q_rope or k_nope and k_rope.
+    """
+    assert len(tensors) == 2
+
+    qk_nope, qk_rope = tensors
+    assert qk_nope.ndim == 3 and qk_rope.ndim == 3
+
+    torch._dynamo.mark_dynamic(qk_nope, 0)
+    torch._dynamo.mark_dynamic(qk_rope, 0)
+
+    return _compiled_cat([qk_nope, qk_rope], dim=dim)
+
+
 @dataclass(frozen=True)
 class NSAIndexerMetadata(BaseIndexerMetadata):
     attn_metadata: NSAMetadata
@@ -942,7 +963,7 @@ class NativeSparseAttnBackend(AttentionBackend):
                         kv_cache, page_table_1_flattened
                     )
                 else:
-                    kv_cache = torch.cat([k, k_rope], dim=-1)
+                    kv_cache = _cat([k, k_rope], dim=-1)
                 page_table_1 = topk_indices
 
             return self._forward_flashmla_sparse(