[Dist Autograd] Functional API for Dist Autograd and Dist Optimizer (#33711)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33711
Fixed #33480
This makes `dist_autograd.backward` and `dist_optimizer.step` functional by making the user explicitly pass in the `context_id` as opposed to relying on the confusing thread_local context_id.
This diff incorporates these API changes and all places where these functions are called.
More concretely, this code:
```
with dist_autograd.context():
# Forward pass.
dist_autograd.backward([loss.sum()])
dist_optim.step()
```
should now be written as follows:
```
with dist_autograd.context() as context_id:
# Forward pass.
dist_autograd.backward(context_id, [loss.sum()])
dist_optim.step(context_id)
```
Test Plan: Ensuring all existing dist_autograd and dist_optimizer tests pass with the new API. Also added a new test case for input checking.
Differential Revision: D20011710
fbshipit-source-id: 216e12207934a2a79c7223332b97c558d89d4d65
