mul(sparse_csr, sparse_csr) using mul(sparse, sparse)
Basic fallback implementation. Let's make this faster once used.
NOTE: This is stacked on top of https://github.com/pytorch/pytorch/pull/74294
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74266
Approved by: https://github.com/pearu, https://github.com/malfet
