Supervised GNNModular tasks

[tonydavis629]

Pull Request Template

Description

This PR enables supervised tasks in GNNModular, and abstracts out the unsupervised edge prediction loss into models.losses.

PR 3339 should be merged first.

Type of change

Please check the option that is related to your PR.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
  • In this case, we recommend to discuss your modification on GitHub issues before creating the PR
• Documentations (modification for documents)

Checklist

• My code follows the style guidelines of this project
  • Run yapf -i <modified file> and check no errors (yapf version must be 0.32.0)
  • Run mypy -p deepchem and check no errors
  • Run flake8 <modified file> --count and check no errors
  • Run python -m doctest <modified file> and check no errors
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New unit tests pass locally with my changes
• I have checked my code and corrected any misspellings

[rbharath]

Did a quick skim and nothing major is jumping out at me, but I will hold off on review until #3339 is merged in

[rbharath]

A few requests for documentation below and we should be good to go

[rbharath]

LGTM, feel free to merge in once CI is clear