[PyTorch FX] Return mapping of qualified names from split_module() (#73564)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73564
While maintaining API backward compatibility, add an optional output parameter to split_module() that returns a mapping from the new qualified names in the modules after split to the old qualified names in the original module
Test Plan:
1. Added a test (test_split_qualname_mapping) to test_fx_experimental.py to check the returned qualname mapping
```
$ python test_fx_experimental.py
...
Ran 1084 tests in 73.464s
OK (skipped=531, expected failures=4)
```
2. Ask test_fx.py to accept split_module's new signature
```
$ python test_fx.py --accept
```
Reviewed By: jamesr66a
Differential Revision: D34541792
fbshipit-source-id: e8ec7e77ec884e4db7cad0c0593e31861c76e42d
(cherry picked from commit d2e5a95a353ee5fb52cdba065f127489e9df47ae)
