[BugFix] Fix FI accuracy issue when used for MLA prefill (#26063)

Signed-off-by: Lucas Wilkinson <lwilkins@redhat.com>
Signed-off-by: Lucas Wilkinson <LucasWilkinson@users.noreply.github.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: mgoin <mgoin64@gmail.com>

vllm/v1/attention/backends/mla/common.py

@@ -1211,13 +1211,18 @@ class MLACommonImpl(MLACommonBaseImpl[M], Generic[M]):
                                    k, v, return_softmax_lse):
         assert isinstance(prefill, FlashInferPrefillMetadata)
         assert prefill.prefill_main is not None
-        return prefill.prefill_main.run(
+        ret = prefill.prefill_main.run(
             q=q,
             k=k,
             v=v,
             return_lse=return_softmax_lse,
         )
 
+        if isinstance(ret, tuple):
+            # Convert from (q_len, num_heads) to (num_heads, q_len)
+            return ret[0], ret[1].transpose(0, 1).contiguous()
+        return ret
+
     def _run_prefill_new_tokens_cudnn(self, prefill: MLACommonPrefillMetadata,
                                       q, k, v, return_softmax_lse):
         assert isinstance(prefill, CudnnPrefillMetadata)
@@ -1260,12 +1265,14 @@ class MLACommonImpl(MLACommonBaseImpl[M], Generic[M]):
     def _run_prefill_context_chunk_fi(self, prefill: MLACommonPrefillMetadata,
                                       chunk_idx: int, q, k, v):
         assert isinstance(prefill, FlashInferPrefillMetadata)
-        return prefill.prefill_chunks[chunk_idx].run(
+        attn_out, lse = prefill.prefill_chunks[chunk_idx].run(
             q=q,
             k=k,
             v=v,
             return_lse=True,
         )
+        # Convert from (q_len, num_heads) to (num_heads, q_len)
+        return attn_out, lse.transpose(0, 1).contiguous()
 
     def _run_prefill_context_chunk_cudnn(self,
                                          prefill: MLACommonPrefillMetadata,