Improve NumPy -> Torch conversions for read-only data

[mdekstrand]

Torch does not support read-only tensors, which can arise in some cases when converting from Arrow or deserializing in Ray.

This PR introduces the safe_tensor function to convert an array to a tensor, and uses it throughout in order to reduce risk of errors from non-modifiable tensors.

[codecov]

Codecov Report

Attention: Patch coverage is 92.59259% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.22%. Comparing base (999c00c) to head (54cf1c1).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/lenskit/data/attributes.py	50.00%	1 Missing ⚠️
src/lenskit/torch.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #680   +/-   ##
=======================================
  Coverage   90.22%   90.22%           
=======================================
  Files         134      134           
  Lines        9260     9271   +11     
=======================================
+ Hits         8355     8365   +10     
- Misses        905      906    +1     

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