Recompute flat_arg_fake_tensors after fakification (#98769)
Summary: This fixes the case when some of the input tensors were
real tensors and fakified in `validate_and_convert_non_fake_tensors`,
but `flat_arg_fake_tensors` would not contain all the inputs
because it was computed before the fakification. We fix this by
recomputing `flat_arg_fake_tensors` after fakification as well.
Test Plan:
python test/dynamo/test_export.py ExportTests.test_mixed_real_and_fake_inputs
Reviewers: Chillee, voznesenskym
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98769
Approved by: https://github.com/voznesenskym
