Snap featurizer

[tonydavis629]

Pull Request Template

Description

This PR implements the featurizer described in Hu, W. et al. Strategies for Pre-training Graph Neural Networks. Preprint at https://doi.org/10.48550/arXiv.1905.12265 (2020). This is a simple featurizer for molecules with its features defined by rdkit.chem objects.

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]

One minor comment but otherwise good to merge in once the CI is green. Let me know once CI looks good and I'll give final thumbs up and merge

[rbharath]

Is this meant to be here? Looks like it could be commented out code or a terse comment. If it's a comment, can you elaborate a bit more so it's clearer?

[rbharath]

One minor question about a potential docs issue

[rbharath]

LGTM, feel free to merge in once CI clear