[MPS] Fix addmm calculation (#94534)
Ignore input when beta is 0, so that `nan` and `inf` will not be propagated.
Case already part of test_mps at https://github.com/pytorch/pytorch/blob/master/test/test_mps.py#L6308
Pull Request resolved: https://github.com/pytorch/pytorch/pull/94534
Approved by: https://github.com/kulinseth
