scGPT padding and tokenization component

[dorien-er]

Changelog

module for padding + tokenizing data for scGPT integration (zero shot or fine-tuning)

Checklist before requesting a review

• [x] I have performed a self-review of my code

• [x] Conforms to the Contributor's guide

• Check the correct box. Does this PR contain:

  • [ ] Breaking changes
  • [x] New functionality
  • [ ] Major changes
  • [ ] Minor changes
  • [ ] Documentation
  • [ ] Bug fixes

• [x] Proposed changes are described in the CHANGELOG.md

• [x] CI tests succeed!