[BugFix] Fix full-cuda-graph illegal memory access in FA3 (#20057)

Signed-off-by: Lucas Wilkinson <lwilkins@redhat.com>

vllm/v1/attention/backends/flash_attn.py

@@ -158,12 +158,13 @@ class FlashAttentionMetadataBuilder(
 
         self.aot_schedule = (get_flash_attn_version() == 3)
         self.use_full_cuda_graph = compilation_config.full_cuda_graph
-        if self.use_full_cuda_graph and not self.aot_schedule:
-            raise ValueError("Full CUDA graph mode requires AOT scheduling, "
-                             "which requires FlashAttention 3.")
-        self.scheduler_metadata = torch.zeros(self.runner.max_num_reqs + 1,
-                                              dtype=torch.int32,
-                                              device=self.runner.device)
+        if self.use_full_cuda_graph:
+            # NOTE(lucas): AOT scheduling not supported in full cuda graph mode
+            #  yet. This is because the scheduler and kernel need to always use
+            #  the same num_splits (which acts as an upper bound with the
+            #  dynamic split scheduler) which is currently heuristically decided
+            #  by the kernel launching code.
+            self.aot_schedule = False
 
         # Sliding window size to be used with the AOT scheduler will be
         # populated on first build() call.
@@ -299,18 +300,6 @@ class FlashAttentionMetadataBuilder(
                                           max_seq_len=max_seq_len,
                                           causal=True)
 
-        if self.use_full_cuda_graph:
-            assert scheduler_metadata is not None
-            n = scheduler_metadata.shape[0]
-            self.scheduler_metadata[:n].copy_(scheduler_metadata,
-                                              non_blocking=True)
-            # NOTE(woosuk): We should zero out the rest of the scheduler
-            # metadata to guarantee the correctness. Otherwise, some thread
-            # blocks may use the invalid scheduler metadata and overwrite the
-            # output buffer.
-            self.scheduler_metadata[n:] = 0
-            scheduler_metadata = self.scheduler_metadata[:n]
-
         attn_metadata = FlashAttentionMetadata(
             num_actual_tokens=num_actual_tokens,
             max_query_len=max_query_len,