Add context manager for conditional rewrites of torch.* to torch._refs.* calls (#81764)
Adds a new context manager `TorchRefsNvfuserCapabilityMode` for conditional rewrite of `torch.*` calls to `torch._refs.*` based on whether the decomposition consisting of prims supports nvFuser execution or not.
A new optional argument for `TorchRefsMode` is added - `should_fallback_fn`, a callable that returns whether the original `torch.foo` or the replacement `torch._refs.foo` should be used.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81764
Approved by: https://github.com/ezyang
