Use default FakeTensorMode when calling module without input (pytorch#16222)

Before, _ExportPassBase set self.tracer.fake_tensor_mode to a default
value, but didn't use it when tracing. This caused operators that only
have fake tensor implementations to crash.


Signed-off-by: Erik Lundell <erik.lundell@arm.com>

Author: Erik-Lundell

backends/arm/test/ops/test_cond.py

@@ -238,8 +238,6 @@ def test_cond_tosa_FP(case: Callable[[], tuple[torch.nn.Module, tuple]]):
     "case",
     test_cases,
     xfails={
-        "zero_args_one_output": "Since the submodules have no input, the tracer fails finding a fake tensor mode,"
-        " and traces the graph with real tensors, which tosa.RESCALE can't handle.",
         "one_arg_and_scalar_one_output": "Incorrect quantization on the scalar.",
         "nested_one_arg_one_output": "Node submodule_0 target submodule_0 references nonexistent attribute submodule_0",
     },

exir/pass_base.py

@@ -1,5 +1,6 @@
 # Copyright (c) Meta Platforms, Inc. and affiliates.
 # All rights reserved.
+# Copyright 2025 Arm Limited and/or its affiliates.
 #
 # This source code is licensed under the BSD-style license found in the
 # LICENSE file in the root directory of this source tree.
@@ -632,14 +633,13 @@ def call(self, graph_module: fx.GraphModule) -> PassResult:
                 ), "Multiple fake tensor mode detected."
                 fake_tensor_mode = i.fake_mode
         if fake_tensor_mode is None:
-            self.tracer.fake_tensor_mode = FakeTensorMode(allow_non_fake_inputs=True)
-            fake_tensor_mode = nullcontext()  # type: ignore[assignment]
+            fake_tensor_mode = FakeTensorMode(allow_non_fake_inputs=True)
             dispatcher_mode = nullcontext()  # type: ignore[assignment]
         else:
             fake_tensor_mode.allow_non_fake_inputs = True
-            self.tracer.fake_tensor_mode = fake_tensor_mode
             dispatcher_mode = enable_python_dispatcher()  # type: ignore[assignment]
-        self.fake_tensor_mode = self.tracer.fake_tensor_mode
+        self.tracer.fake_tensor_mode = fake_tensor_mode
+        self.fake_tensor_mode = fake_tensor_mode
 
         with fake_tensor_mode, dispatcher_mode:  # type: ignore[assignment, union-attr]
             result = self.call_submodule(graph_module, tuple(inputs))