refactor: update f-string for swa.utils.py (#68718)
Summary:
_ Update some old-style formats to f-string, for whole and coherent consistency.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68718
Reviewed By: jbschlosser
Differential Revision: D32593746
Pulled By: albanD
fbshipit-source-id: fcc17958f8af6a3260beca883bc1065f019dcf0e
