minor: global workspace buffer for trtllm-gen mha from flashinfer (#8952)

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

@@ -25,6 +25,9 @@ if TYPE_CHECKING:
 # Constants
 DEFAULT_WORKSPACE_SIZE_MB = 128  # Memory workspace size in MB
 
+# Reuse this workspace buffer across all TRTLLM MHA wrappers
+global_workspace_buffer = None
+
 
 @dataclass
 class TRTLLMMHAMetadata:
@@ -69,9 +72,15 @@ class TRTLLMHAAttnBackend(FlashInferAttnBackend):
 
         # Workspace allocation
         self.workspace_size = DEFAULT_WORKSPACE_SIZE_MB * 1024 * 1024
-        self.workspace_buffer = torch.empty(
-            self.workspace_size, dtype=torch.int8, device=self.device
-        )
+        # Allocate buffers
+        global global_workspace_buffer
+        if global_workspace_buffer is None:
+            global_workspace_buffer = torch.empty(
+                self.workspace_size,
+                dtype=torch.uint8,
+                device=model_runner.device,
+            )
+        self.workspace_buffer = global_workspace_buffer
 
         # CUDA graph state
         self.decode_cuda_graph_metadata = {}