Improve pytorch type hints (Dataloader, trig functions)
Summary:
This is to fix Pyre errors in our applications:
* calling `tensor.cos()` etc.
* creating a data loader with batch sampler that is `List[List[int]]`.
Test Plan: TODO: rebase the diffs and run Pyre.
Reviewed By: ejguan
Differential Revision: D31309564
fbshipit-source-id: 1c6f3070d7570260de170e2fe2153d277b246745
