[Perf] Add decode full-graph support to FlashInfer-MLA backend (#26313)

Signed-off-by: Benjamin Chislett <bchislett@nvidia.com>

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

 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
-from typing import Optional, Union
+from typing import ClassVar, Optional, Union
 
 import torch
 from flashinfer.decode import trtllm_batch_decode_with_kv_cache_mla
@@ -12,13 +12,20 @@ from vllm.v1.attention.backends.mla.common import (
     MLACommonBackend,
     MLACommonImpl,
     MLACommonMetadata,
+    MLACommonMetadataBuilder,
 )
+from vllm.v1.attention.backends.utils import AttentionCGSupport
 
 logger = init_logger(__name__)
 
 FLASHINFER_MLA_WORKSPACE_BUFFER_SIZE = 128 * 1024 * 1024
 
 
+class FlashInferMLAMetadataBuilder(MLACommonMetadataBuilder[MLACommonMetadata]):
+    # enable full CUDA Graph support for decode-only capture
+    cudagraph_support: ClassVar[AttentionCGSupport] = AttentionCGSupport.UNIFORM_BATCH
+
+
 class FlashInferMLABackend(MLACommonBackend):
     @staticmethod
     def get_name() -> str:
@@ -28,6 +35,10 @@ class FlashInferMLABackend(MLACommonBackend):
     def get_impl_cls() -> type["FlashInferMLAImpl"]:
         return FlashInferMLAImpl
 
+    @staticmethod
+    def get_builder_cls() -> type["FlashInferMLAMetadataBuilder"]:
+        return FlashInferMLAMetadataBuilder
+
 
 g_fi_workspace = torch.zeros(
     FLASHINFER_MLA_WORKSPACE_BUFFER_SIZE,