Allow disabling cache in autocast (automatic mixed precision) (#63552)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63552
In this PR, we want to exclude these 2 cases in the `Autocast` weight cache usages:
- Using `torch.jit.trace` under the `Autocast`
As report in https://github.com/pytorch/pytorch/issues/50231 and several other discussions, using `torch.jit.trace` under the `Autocast`, the trace process would hit Autocast's weight cache and fails. So we should disable weight cache under the trace process.
- Using `Autocast` with `Grad mode`
- Usually we are using `Grad mode` for training. Since in the training phase, the weight will change in every step. So we doesn't need to cache the weight.
- For the recommended `Autocast` training case in the [doc](https://pytorch.org/docs/stable/amp.html), `Autocast` will clear the cache every step leaving the context. We should disable it to save the clear operations.
```
model = Net().cuda()
optimizer = optim.SGD(model.parameters(), ...)
for input, target in data:
optimizer.zero_grad()
with autocast():
output = model(input)
loss = loss_fn(output, target)
loss.backward()
optimizer.step()
```
Test Plan: Imported from OSS
Reviewed By: mrshenli
Differential Revision: D30644913
Pulled By: ezyang
fbshipit-source-id: ad7bc87372e554e7aa1aa0795e9676871b3974e7
