[PyTorch] Save a refcount bump in meta functions for addmm and mm (#59063)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59063
`TensorMeta::maybe_get_output()` returns `const Tensor&`, no need to copy the Tensor..
ghstack-source-id: 130044287
Test Plan: CI
Reviewed By: ngimel
Differential Revision: D28735225
fbshipit-source-id: f2bdf39b28de245ec4664718490e7e0b36bc8819
