feat: Remove support for PyTorch backend

[matthewfeickert]

Description

• Remove support for PyTorch from the codebase.
  • This is a breaking change.
  • Remove the PyTorch backend and optimize/opt_pytorch.py.
• Remove use of PyTorch from the tests.
  • Remove PyTorch lower bounds from tests/constraints.txt.
  • Remove comparison of tensor library backends in tests/test_backend_consistency.py as there is only NumPy and JAX now.
• Remove all docs and examples that mention or use PyTorch.
• Remove mentions of PyTorch in all parts of the website and REAMDE except for historical purposes like the release notes, JOSS paper, and notes about early pyhf research in FAQs.
  • For abstract use term "automatic differentiation".
• Update NumPy backend docstrings to accommodate NumPy v2.0 returns.

Factorizes out parts of PR #2566 and removes additional documentation pieces.

Affected Issues

• Resolves move to /add TF and PyTorch BFGS optimizers #583
• Resolves Enable CUDA support for PyTorch backend #1244
• Resolves pytorch TPU XLA support  #1248
• Resolves Track and update changes to percentile in PyTorch and JAX backends #1693
• Resolves Warnings with pytorch backend and simplemodels #1946
• Resolves Fix DeprecationWarning for PyTorch for NumPy 2.0 #2554
• Resolves Include jax in list of supported backends in InvalidBackend exception #2605

Please describe the purpose of this pull request in some detail. Reference and link to any relevant issues or pull requests.

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Remove support for PyTorch from the codebase.
   - This is a breaking change.
   - Remove the PyTorch backend and optimize/opt_pytorch.py.
* Remove use of PyTorch from the tests.
   - Remove PyTorch lower bounds from tests/constraints.txt.
   - Remove comparison of tensor library backends in
     tests/test_backend_consistency.py as there is only NumPy and JAX now.
* Remove all docs and examples that mention or use PyTorch.
* Remove mentions of PyTorch in all parts of the website and REAMDE except
  for historical purposes like the release notes, JOSS paper, and notes
  about early pyhf research in FAQs.
   - For abstract use term "automatic differentiation".
* Update NumPy backend docstrings to accommodate NumPy v2.0 returns.

Co-authored-by: Giordon Stark <kratsg@gmail.com>

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.23%. Comparing base (7eda55b) to head (d9c025d).
⚠️ Report is 36 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2611      +/-   ##
==========================================
- Coverage   98.25%   98.23%   -0.03%     
==========================================
  Files          67       65       -2     
  Lines        4360     4193     -167     
  Branches      600      591       -9     
==========================================
- Hits         4284     4119     -165     
+ Misses         46       45       -1     
+ Partials       30       29       -1     

Flag	Coverage Δ
contrib	97.97% <100.00%> (+0.08%)	⬆️
doctest	98.09% <100.00%> (-0.03%)	⬇️
unittests-3.10	96.27% <100.00%> (+0.01%)	⬆️
unittests-3.11	96.27% <100.00%> (+0.01%)	⬆️
unittests-3.12	96.27% <100.00%> (?)
unittests-3.8	96.27% <100.00%> (+0.01%)	⬆️
unittests-3.9	96.32% <100.00%> (+0.01%)	⬆️

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[matthewfeickert]

Note that docstrings for NumPy got updated as NumPy can now be NumPy v2.0 and so the docstest changed.