add onnxscript dependency and split up PyTorch tests

[jmelburg]

ONNXModel.from_pytorch requires onnxscript. This wasn't picked up before because the relevant tests were skipped:

@pytest.mark.skipif(not _pytorch_available(), reason="torch not installed")

Now that #780 pulls PyTorch into the tests dependencies, the tests trigger, surfacing the issue.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.45%. Comparing base (3275cc6) to head (a335a52).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #822      +/-   ##
==========================================
- Coverage   89.29%   88.45%   -0.84%     
==========================================
  Files         128      128              
  Lines       10665    10665              
==========================================
- Hits         9523     9434      -89     
- Misses       1142     1231      +89     

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