tests(python): Make torch install CI-only by default

[alexander-beedie]

Streamlining the default dev environment...

• Moves torch package from "requirements-dev.txt" to new "requirements-ci.txt".
• Ensures that typing lint passes for polars.ml.torch with/without local torch install.
• Renames the third_party_integration marker to ci_only to better describe its purpose.
• Adds a make requirements-all command that installs the usual dev packages and the "CI only" packages.
• Makes the top-level torch import in the "test_to_torch" unit tests lazy-load so that optional absence of the package doesn't cause pytest to complain when run locally (the CI runner will always have it installed, so no issue there).

We might want to make a couple of other packages CI-only by default, but for now this only covers the new torch integrations.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 80.93%. Comparing base (eb7f939) to head (67deca2).
Report is 3 commits behind head on main.

Files	Patch %	Lines
py-polars/polars/ml/torch.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16058      +/-   ##
==========================================
- Coverage   80.97%   80.93%   -0.05%     
==========================================
  Files        1386     1386              
  Lines      178382   178382              
  Branches     3059     3059              
==========================================
- Hits       144446   144372      -74     
- Misses      33448    33522      +74     
  Partials      488      488              

Flag	Coverage Δ
python	74.49% <0.00%> (-0.03%)	⬇️
rust	78.15% <0.00%> (-0.01%)	⬇️

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[ritchie46]

Thanks Alex!