Add support to pass a callable as padding mode

[mueller-franzes]

Fixes #957.

Description It fixes the KeyError message when a function is passed as padding_mode in the Pad class.

Checklist

• [x] I have read the CONTRIBUTING docs and have a developer setup (especially important are pre-commitand pytest)
• [x] Non-breaking change (would not break existing functionality)
• [ ] Breaking change (would cause existing functionality to change)
• [x] Tests added or modified to cover the changes
• [x] Integration tests passed locally by running pytest
• [ ] In-line docstrings updated
• [ ] Documentation updated, tested running make html inside the docs/ folder
• [x] 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[bot]]

Codecov Report

Merging #959 (1539c1c) into main (6338270) will increase coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #959      +/-   ##
==========================================
+ Coverage   86.19%   86.22%   +0.03%     
==========================================
  Files          92       92              
  Lines        5801     5802       +1     
==========================================
+ Hits         5000     5003       +3     
+ Misses        801      799       -2     

Impacted Files	Coverage Δ
...rc/torchio/transforms/preprocessing/spatial/pad.py	95.34% <100.00%> (+4.87%)	:arrow_up:

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[fepegar]

Thanks for your contribution, @mueller-franzes!

@all-contributors please add @mueller-franzes for code

[allcontributors[bot]]

@fepegar

I couldn't determine any contributions to add, did you specify any contributions? Please make sure to use valid contribution names.

I've put up a pull request to add @mueller-franzes! :tada: