Prevent legacy_load when weights_only=True (correctly) (#145020)

Only prevent `legacy_load` (.tar format removed in https://github.com/pytorch/pytorch/pull/713), not the whole of `_legacy_load` (.tar format + _use_new_zipfile_serialization=False)

Differential Revision: [D68301405](https://our.internmc.facebook.com/intern/diff/D68301405)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/145020
Approved by: https://github.com/kit1980, https://github.com/albanD

(cherry picked from commit 0eda02a94c754e2256ff1701bcc03c40ece2bbef)

Co-authored-by: Mikayla Gawarecki <mikaylagawarecki@gmail.com>