[WIP] Fix cpp grad accessor API (#40887)
Summary:
Update the API to access grad in cpp to avoid unexpected thread safety issues.
In particular, with the current API, a check like `t.grad().defined()` is not thread safe.
- This introduces `t.mutable_grad()` that should be used when getting a mutable version of the saved gradient. This function is **not** thread safe.
- The `Tensor& grad()` API is now removed. We could not do a deprecation cycle as most of our call side use non-const Tensors that use the non-const overload. This would lead to most calls hitting the warning. This would be too verbose for all the users.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40887
Reviewed By: ezyang
Differential Revision: D22343932
Pulled By: albanD
fbshipit-source-id: d5eb909bb743bc20caaf2098196e18ca4110c5d2
