Generalize sparse matmul

[mtfishman]

This helps with cases where the unstored elements aren't well defined to generate, for example block sparse matrix multiplication where the block type is abstract (ITensor/BlockSparseArrays.jl#150).

This also fixes an issue with printing sparse arrays where the elements are GPU arrays.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.70%. Comparing base (6738bee) to head (ec22c31).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #62      +/-   ##
==========================================
+ Coverage   75.76%   76.70%   +0.93%     
==========================================
  Files           8        8              
  Lines         586      601      +15     
==========================================
+ Hits          444      461      +17     
+ Misses        142      140       -2     

Flag	Coverage Δ
docs	35.72% <81.25%> (+1.13%)	⬆️

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