Avoid mat1 references in mm_mat1_backward (#45777)
Summary:
Avoiding references to `mat1` in `mm_mat1_backward` is a first step to solving issue https://github.com/pytorch/pytorch/issues/42371
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45777
Reviewed By: malfet
Differential Revision: D24347967
Pulled By: albanD
fbshipit-source-id: f09a8149d9795481b5ed5b48fdd0e598ba027d0b
