V2: Add more example notebooks

[kevingreenman]

Continued from #676, which was closed when I moved the branch from the chemprop organization to my personal fork. Please see discussion and unresolved action items there.

[kevingreenman]

per the checklist in #556 / #676, we should also consider adding notebooks for the following, many of which were requested by MLPDS members:

• active learning
• attribution ([TODO]: v2 attribution examples #698)
• transfer learning
• custom loss functions
• HPO
• custom featurizers and interoperability (@hwpang, [TODO]: Add example notebook(s) for interoperability between chemprop and other libraries #804)

[kevingreenman]

• custom featurizers and interoperability (@hwpang, [TODO]: Add example notebook(s) for interoperability between chemprop and other libraries #804)

@hwpang when we add a new example notebook for custom featurizers, we should be sure we have a good example of how to use the rdkit features that were easy to do in the v1 CLI, as requested in #843 and #849

[KnathanM]

My PR #834 added a very small example of writing custom loss functions. Is that enough to check off that box, or do we want a longer worked example?

[kevingreenman]

My PR #834 added a very small example of writing custom loss functions. Is that enough to check off that box, or do we want a longer worked example?

I think maybe a little more detail than what we have so far in #834, but that's a good start

[JacksonBurns]

Commenting that the linked issue removed just above my comment was un-linked because this PR will no longer attempt to run the notebooks on Colab/Binder. See that issue for why.