Remove redundant `found_inf` recompute from `_step_supports_amp_unscaling` path (#98620)
following https://github.com/pytorch/pytorch/pull/97415#issuecomment-1499787115.
Rel: https://github.com/pytorch/pytorch/pull/98613
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98620
Approved by: https://github.com/janeyx99
