[ONNX] Skip doctest `torch.onnx._internal.fx` if ImportError (#95686)
Need to use `exclude` to skip the module altogether. Because xdoctest
triggers `ImportError` when trying to import the module. So the whole
test fails regardless if skip was added in the docstring.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/95686
Approved by: https://github.com/kit1980, https://github.com/titaiwangms