[BugFix] fix VLLM_USE_STANDALONE_COMPILE=0 (#38015)

Signed-off-by: Richard Zou <zou3519@gmail.com>

vllm/compilation/compiler_interface.py

@@ -632,6 +632,23 @@ class InductorAdaptor(CompilerInterface):
             )
             stack.enter_context(_patch_constrain_to_fx_strides())
 
+            # Clear the tracing context before calling compile_fx.
+            # vLLM calls compile_fx from within a PiecewiseCompileInterpreter
+            # that runs under Dynamo's tracing context. The tracing context
+            # has a FakeTensorMode from Dynamo, but the example inputs for
+            # this subgraph have fake tensors from a different FakeTensorMode.
+            # compile_fx's _compile_fx_main calls detect_fake_mode() which
+            # asserts all FakeTensorModes match, causing a crash.
+            # Clearing the tracing context lets compile_fx create its own.
+            saved_tracing_context = torch._guards.TracingContext.try_get()
+            if saved_tracing_context is not None:
+                torch._guards._TLS.tracing_context = None
+
+                def _restore_tracing_context():
+                    torch._guards._TLS.tracing_context = saved_tracing_context
+
+                stack.callback(_restore_tracing_context)
+
             compiled_graph = compile_fx(
                 graph,
                 example_inputs,