[Experiment] Kernel for sparse matrix-dense matrix multiplication

[mercush]

Proposed changes

Feature: added function metal kernel sparse_matmul_csr for sparse matrix-dense matrix multiplication. The function takes as input a sparse matrix (represented as three arrays representing the row pointers, the column indices, and the values at the nonzero entries) and a dense matrix represented as a dense array with two dimensions. I have implemented the backends for metal and for the cpu but not cuda.

Please let me know if there is any documentation I should update.

Checklist

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• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)