Add flexible Torch matrix factorizer

[mdekstrand]

This is the long-awaited change to bring a flexible PyTorch matrix factorization implementation to LensKit (#203).

[codecov]

Codecov Report

Attention: Patch coverage is 97.82609% with 8 lines in your changes missing coverage. Please review.

Project coverage is 91.03%. Comparing base (b7b5b03) to head (1e2e5f4).
Report is 35 commits behind head on main.

Files with missing lines	Patch %	Lines
src/lenskit/flexmf/_model.py	92.30%	4 Missing ⚠️
src/lenskit/data/relationships.py	33.33%	2 Missing ⚠️
src/lenskit/flexmf/_base.py	98.11%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #668      +/-   ##
==========================================
+ Coverage   90.58%   91.03%   +0.45%     
==========================================
  Files         126      132       +6     
  Lines        8717     9078     +361     
==========================================
+ Hits         7896     8264     +368     
+ Misses        821      814       -7     

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