store batch_shape as torch.Size object in MultivariateStudentT distribution

[flo-schu]

resolves #3098
according to suggestions by @fritzo

Since this is my first contribution here, I'd be happy if you could point out, things I should improve in the PR

[fritzo]

happy if you could point out, things I should improve in the PR

Looks great, it's super simple. One thing we do is add a "Tested" section to PR descriptions, but this would would just look like:

Tested

• refactoring is covered by existing tests

[fritzo]

Could you make format to fix the long line?

[flo-schu]

Could you make format to fix the long line?

I assume there is a shortcut to what I did?

[fritzo]

I assume there is a shortcut to what I did?

All I see is the failed lint stage (LMK if you have trouble accessing this) where black complains. I don't know what exactly it's complaining about (guessing a long line), but whatever it's complaining about you should be able to fix via make format or simply black .. We do mention formatting in CONTRIBUTING.md and if you can offer any improvements to contributing docs, we welcome those contributions too 🙂