Expand sparse.softmax zero nnz tests to cover cases of previously reported FPE. (#95646)
- Test cases with zero `nnz` added for `sparse.log_softmax`.
- Test cases with zero `nnz` for both `sparse.log_softmax` and
`torch.sparse_softmax` expanded to cover the backward pass.
These test additions prove resolution to #95371 and #82107.
Fixes #82107 #95371
Pull Request resolved: https://github.com/pytorch/pytorch/pull/95646
Approved by: https://github.com/cpuhrsch, https://github.com/pearu, https://github.com/nikitaved
