Dev minor

[lxuechen]

Minor changes

• state requirements in README,
• trim latent_sde.py in examples,
• use torch.split for flat_to_shape, since it's slightly faster than tensor slicing, and
• update documentation for sdeint and sdeint_adjoint.

This PR is mostly trigger by #61, adding some minor changes that I had in my in mind for a while.

Even though logqp is removed from the documentation, I still plan to support logqp in the API, merely for good backward compatibility. The concrete plan there is to have a wrapper that creates a new set of f and g from existing ones in the SDE object passed in. That should come in a future PR and shouldn't be complicated as a change.

[lxuechen]

Happy to squash and merge this? @patrick-kidger

[patrick-kidger]

Yep, LGTM. (And welcome back, btw.)