Fix flaky Dynamo export tests (#96488)
Planning to do a full writeup later. The short story is, sometimes the following chain of events happens:
1. We turn on Dynamo's custom frame handler
2. GC triggers (and all of the finalizers run under Dynamo)
3. GC hits a GeneratorExit frame
4. You end up in the custom frame handler with throw_flag == TRUE and PyErr_Occurred() != NULL
If this happens and we blindly call into other Python functions (like the Python callback), the executed Python code will immediately raise an exception (because there's already an ambient exception set.) This is very, very confusing. The fix is to defer to the regular handler when throw_flag is TRUE.
I triggered this locally with
```
PYTHONUNBUFFERED=1 pytest test/dynamo/test_dynamic_shapes.py -k 'Unspec and export and not dupes and not reorder' -v -x -s
```
But I also have some tests which trigger the problem synthetically.
Fixes https://github.com/pytorch/pytorch/issues/93781
Signed-off-by: Edward Z. Yang <ezyang@meta.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96488
Approved by: https://github.com/albanD
