fix misspelling in swa_utils.pyi (#51608)
Summary:
Change `avg_fun -> avg_fn` to match the spelling in the `.py` file.
(`swa_utils.pyi` should match `swa_utils.py`)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51608
Reviewed By: glaringlee
Differential Revision: D26224779
Pulled By: zou3519
fbshipit-source-id: 01ff7173ba0a996f1b7a653438acb6b6b4659de6
