[BlockSparseArrays] Sparse and block sparse matrix multiplication

[mtfishman]

This adds support for sparse and block sparse matrix multiplication.

TODO:

• Test matrix multiplication of DiagonalArray and SparseArrayDOK.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0ff3110) 84.03% compared to head (888512f) 53.82%.

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Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1278       +/-   ##
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- Coverage   84.03%   53.82%   -30.22%     
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  Files         100       99        -1     
  Lines        8540     8487       -53     
===========================================
- Hits         7177     4568     -2609     
- Misses       1363     3919     +2556     

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[emstoudenmire]

Great. Does this already use the sparse tensor type in the way you were saying, with the 'elements' being dense tensors/matrices?

[mtfishman]

Yeah, that's been the design of the BlockSparseArray type in the BlockSparseArrays library.