[ONNX] Passes to reuse existing fake mode if possible (#105764)
Fixes #105467, namely the need of setting `aten_graph=True` in `_dynamo.export`
to make fake mode onnx exporter work.
Previously, `make_fx` called by passes always create new fake mode. Hence it is
missing out info from `shape_env` recorded during dynamo export. This PR tries
to check and fetch existing fake mode from graph node meta.
Also enable python dispatcher context when calling `make_fx`. This is done in
`_dynamo.export(aten_graph=True)` but was missing in our passes.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105764
Approved by: https://github.com/titaiwangms
