Indexing a 4D FrameBatch: Should this return a Frame or a 3D FrameBatch?

[NicolasHug]

Opening this issue so we don't forget to discuss this: #283 (comment)

For now, indexing a 4D FrameBatch returns a (3D) Frame. We could instead return a 3D FrameBatch.

I suggest we first land #284, then I'll submit a PR that makes the change of returning a 3D FrameBatch instead - it'll be easier to evaluate the UX changes that this involves.

CC @ahmadsharif1