[BugFix] bugfix for Flash Attention MLA with full cuda graph IMA following pr-25490 (#27128)

Signed-off-by: qqma <qqma@amazon.com>
Co-authored-by: qqma <qqma@amazon.com>

Author: Daisy-Ma-coder

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

@@ -89,10 +89,9 @@ def __init__(
         self.use_full_cuda_graph = (
             self.compilation_config.cudagraph_mode.has_full_cudagraphs()
         )
+        self.max_cudagraph_size = self.compilation_config.max_capture_size
 
         if self.use_full_cuda_graph and self.fa_aot_schedule:
-            self.max_cudagraph_size = self.compilation_config.max_capture_size
-
             if self.max_cudagraph_size > 992:
                 # This condition derives from FA3's internal heuristic.
                 # TODO(woosuk): Support larger cudagraph sizes.
@@ -114,7 +113,14 @@ def __init__(
             self.max_num_splits = 1
 
     def _schedule_decode(
-        self, num_reqs, cu_query_lens, max_query_len, seqlens, max_seq_len, causal
+        self,
+        num_reqs,
+        cu_query_lens,
+        max_query_len,
+        seqlens,
+        max_seq_len,
+        causal,
+        max_num_splits,
     ):
         if self.fa_aot_schedule:
             return get_scheduler_metadata(
@@ -130,7 +136,7 @@ def _schedule_decode(
                 page_size=self.page_size,
                 cu_seqlens_q=cu_query_lens,
                 causal=causal,
-                num_splits=self.max_num_splits,
+                num_splits=max_num_splits,
             )
         return None
 
@@ -148,17 +154,25 @@ def _build_decode(
         max_query_len = query_lens_cpu.max().item()
         max_seq_len = seq_lens_device.max().item()
 
+        # For Flash Attention MLA + full cudagraph
+        max_num_splits = 0
+        if self.use_full_cuda_graph and num_decode_tokens <= self.max_cudagraph_size:
+            # NOTE(woosuk): Setting num_splits > 1 may increase the memory
+            # usage, because the intermediate buffers of size [num_splits,
+            # num_heads, num_tokens, head_size] are allocated. Therefore,
+            # we only set num_splits when using cuda graphs.
+            max_num_splits = self.max_num_splits
+
         scheduler_metadata = self._schedule_decode(
             num_reqs=seq_lens_cpu.numel(),
             cu_query_lens=query_start_loc_device,
             max_query_len=max_query_len,
             seqlens=seq_lens_device,
             max_seq_len=max_seq_len,
             causal=True,
+            max_num_splits=max_num_splits,
         )
 
-        # For FA3 + full cudagraph
-        max_num_splits = 0
         if self.use_full_cuda_graph and scheduler_metadata is not None:
             n = scheduler_metadata.shape[0]
             # Ensure the persistent buffer is large enough
@@ -174,13 +188,6 @@ def _build_decode(
             self.scheduler_metadata[n:] = 0
             scheduler_metadata = self.scheduler_metadata[:n]
 
-            if num_decode_tokens <= self.max_cudagraph_size:
-                # NOTE(woosuk): Setting num_splits > 1 may increase the memory
-                # usage, because the intermediate buffers of size [num_splits,
-                # num_heads, num_tokens, head_size] are allocated. Therefore,
-                # we only set num_splits when using cuda graphs.
-                max_num_splits = self.max_num_splits
-
         if vllm_is_batch_invariant():
             max_num_splits = 1