sparse.mm.backward: fix for non-contiguous grad values on CPU (#106127)
Fixes https://github.com/pytorch/pytorch/issues/102493.
The problem was that the backward implementation assumed inputs to be contiguous.
This might supersede https://github.com/pytorch/pytorch/pull/104520.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106127
Approved by: https://github.com/cpuhrsch
