[python] deprecate PyTorch loader prototype

[mlin]

The PyTorch loader built-in to the cellxgene-census API is "graduating" as TileDB-SOMA-ML. This adds runtime+docs deprecation notices (leaving it functional for now), and revises the tutorial notebook to use TileDB-SOMA-ML.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.43%. Comparing base (acfde71) to head (53f26c0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1365   +/-   ##
=======================================
  Coverage   92.43%   92.43%           
=======================================
  Files          83       83           
  Lines        6739     6741    +2     
=======================================
+ Hits         6229     6231    +2     
  Misses        510      510           

Flag	Coverage Δ
unittests	92.43% <100.00%> (+<0.01%)	⬆️

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