[ONNX] Exclude FXSymbolicTracer from _assert_fake_tensor_mode (#107712)
Previous to this PR, `_assert_fake_tensor_mode` checks all of exporting tracer that they enable fake mode "from" exporter API whenever they have fake tensors in args/buffers/weights. However, FXSymbolicTracer doesn't use exprter API to create fake mode, so it hits the raise RuntimeError everytime we run it.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107712
Approved by: https://github.com/BowenBao
