[Feature] Add Langevin Dynamics Sampler

[natolambert]

The score-based generative models sometimes us a Langevin Dynamics step to de-noise samples; see NCSN. In the diffusion model for generating molecule conformations, GeoDiff, this step function had the best results (even though it is simpler than the score-based schedulers currently in the repo).

It could be worth adding to the repo, likely after the branch #54 is added.

cc @MinkaiXu

[anton-l]

@natolambert would you like to implement it, if still relevant to GeoDiff?

[natolambert]

@anton-l I got GeoDiff working without it (good enough results for colab). Let me see if this is easy to implement now that I know more.

[natolambert]

Okay, found some pointers. This seems like it would be one of the simplest Schedulers.

• Seems like the NSCN paper was the one to popularize this.
• The implementation in GeoDiff is here (a little different due to the modality).

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[natolambert]

Initial progress on this is done in #1032.

The things that need to be done are:

• integration tests (there is a sketch in the PR)
• changing the API to index discrete sigmas rather than the list of sigmas (like SDE VE)
• make timesteps continuous (not 100% sure on this)
• showcase examples in a colab / another form to verify functionality.

Maybe @MinkaiXu -- expressed interest.

[MinkaiXu]

Yes, glad to contribute @natolambert. Maybe we can find time to chat about this later next week.