Move `process_batch` to an overridable method

[roomrys]

Description

Previously, process_batch was a local function of the Predictor._predict_generator method. What if a certain predictor wants to process_batch a little differently?

This PR moves the process_batch function to an attribute of Predictor, allowing it to be overridden by any Predictor subclasses.

Types of changes

• [ ] Bugfix
• [ ] New feature
• [ ] Refactor / Code style update (no logical changes)
• [ ] Build / CI changes
• [ ] Documentation Update
• [x] Other (flexibility)

Does this address any currently open issues?

[list open issues here]

Outside contributors checklist

• [ ] Review the guidelines for contributing to this repository
• [ ] Read and sign the CLA and add yourself to the authors list
• [ ] Make sure you are making a pull request against the develop branch (not main). Also you should start your branch off develop
• [ ] Add tests that prove your fix is effective or that your feature works
• [ ] Add necessary documentation (if appropriate)

Thank you for contributing to SLEAP!

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[codecov[bot]]

Codecov Report

Merging #1525 (98e77c7) into develop (ed77b49) will decrease coverage by 0.01%. The diff coverage is 93.33%.

@@             Coverage Diff             @@
##           develop    #1525      +/-   ##
===========================================
- Coverage    73.39%   73.39%   -0.01%     
===========================================
  Files          134      134              
  Lines        23961    23961              
===========================================
- Hits         17586    17585       -1     
- Misses        6375     6376       +1     

Files	Coverage Δ
sleap/nn/inference.py	81.25% <93.33%> (ø)

... and 1 file with indirect coverage changes

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