don't attempt to multiply by a sparse matrix (#18737)
Summary:
Tested by running the script in #16562 , and there was no error.
Then:
```
>>> print(mat.grad)
tensor([[1., 2., 3.],
[1., 2., 3.],
[1., 2., 3.]])
```
which is correct.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18737
Differential Revision: D14773078
Pulled By: umanwizard
fbshipit-source-id: 8aa36eb6f6aa104263a467d9ac91d61b3bfd05f5
