[docs][1.5] Update distributed autograd note (#34657)
Summary:
- Update API calls `backward` and `optim.step` now that we require `context_id`
- Add notes to clarify purpose of distributed autograd context (this was a source of confusion in some feedback)
- Add note that details why optimizer requires context_id
- Clearly specify that we don't have SMART mode yet
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34657
Differential Revision: D20427667
Pulled By: rohan-varma
fbshipit-source-id: 5f8a3539ccf648a78e9e9a0dfdfe389c678b1606
