Lightning example for CoxPH loss

[rohanshad]

Minimal example of CoxPH loss working with pytorch lightning in lightning_coxph.py. I've separately built this into my other scripts with integration within the torch lightning test step logic for easier logging. I decided not to do that here since that requires dataset specific transformations / logging with tensorboard.

Made some other small changes to docstrings too.

[havakv]

Great work! Ideally one would have created branch with only changes relative to refactor_out_torchtuples, as now there are some duplicate commits. I don't think that is a big issue here, but something to think about in the future as a clean commit history is always beneficial (git rebase is a great tool for this).