Improve sparse CSR support

[mberr]

At least on CPU, CSR matrices tend to be faster to multiply with a dense matrix.

Also sets the torch minimum version to 1.11

[codecov-commenter]

Codecov Report

Merging #15 (4b515fb) into main (91b7b36) will increase coverage by 0.27%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
+ Coverage   85.90%   86.18%   +0.27%     
==========================================
  Files           4        4              
  Lines         149      152       +3     
  Branches       28       30       +2     
==========================================
+ Hits          128      131       +3     
  Misses         11       11              
  Partials       10       10              

Impacted Files	Coverage Δ
src/torch_ppr/utils.py	87.96% <100.00%> (+0.34%)	⬆️

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