Improve torch.cuda.amp type hints (#108630)
Fixes #108629
1. Add the following to their modules' `__all__` so that pyright considers them to be publicly exported:
* [`torch.autocast`](https://pytorch.org/docs/stable/amp.html#torch.autocast)
* [`torch.cuda.amp.GradScaler`](https://pytorch.org/docs/stable/amp.html#torch.cuda.amp.GradScaler)
* [`torch.cuda.amp.autocast`](https://pytorch.org/docs/stable/amp.html#torch.cuda.amp.autocast)
* [`torch.cuda.amp.custom_fwd`](https://pytorch.org/docs/stable/amp.html#torch.cuda.amp.custom_fwd)
* [`torch.cuda.amp.custom_bwd`](https://pytorch.org/docs/stable/amp.html#torch.cuda.amp.custom_bwd)
2. Add `overload`s for `torch.cuda.amp.GradScaler.scale` to differentiate when a `torch.Tensor` is returned vs. an `Iterable[torch.Tensor]` is returned based on the type of the `outputs` parameter.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/108630
Approved by: https://github.com/ezyang
