Add mechanism to turn any RAII guard into a Python Context Manager (#102037)
This PR:
- adds a mechanism to turn any RAII guard into a Python Context Manager
- turns ExcludeDispatchKeyGuard into a context manager, and purges usages
of the older torch._C.ExcludeDispatchKeyGuard from the codebase.
The mechanism is that given a RAII guard, we construct a context
manager object that holds an optional guard. When we enter the context
manager we populate the guard, when we exit we reset it.
We don't delete torch._C.ExcludeDispatchKeyGuard for BC reasons (people
are using it in fbcode). If this code actually sticks
(it is using C++17 and that worries me a bit), then I'll apply the
change to other RAII guards we have, otherwise, we can write our own
std::apply.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/102037
Approved by: https://github.com/ezyang, https://github.com/bdhirsh
