Fix CropOrPad incorrectly storing bounds

[fepegar]

Fixes #757.

Description
The current implementation of CropOrPad incorrectly stores some data in self, causing the bug described in:

• CropOrPad self.bounds_parameters is set only for first image. #757

Checklist

• I have read the CONTRIBUTING docs and have a developer setup (especially important are pre-commitand pytest)
• Non-breaking change (would not break existing functionality)
• Breaking change (would cause existing functionality to change)
• Tests added or modified to cover the changes
• Integration tests passed locally by running pytest
• In-line docstrings updated
• Documentation updated, tested running make html inside the docs/ folder
• This pull request is ready to be reviewed
• If the PR is ready and there are multiple commits, I have squashed them and force-pushed

[codecov]

Codecov Report

Merging #760 (a9d0ef2) into main (b6ba3d1) will decrease coverage by 0.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #760      +/-   ##
==========================================
- Coverage   94.03%   93.88%   -0.15%     
==========================================
  Files          72       72              
  Lines        4492     4499       +7     
==========================================
  Hits         4224     4224              
- Misses        268      275       +7     

Impacted Files	Coverage Δ
...io/transforms/preprocessing/spatial/crop_or_pad.py	96.72% <100.00%> (-0.08%)	⬇️
torchio/utils.py	75.92% <100.00%> (+1.16%)	⬆️
torchio/transforms/transform.py	94.40% <0.00%> (-2.62%)	⬇️

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