Add new dispatch keys for Fake Tensor and Deferred Module Initialization
Thanks to @bdhirsh's work, we now have room for new dispatch keys in `DispatchKey` enum. This PR adds two new keys for out-of-core [Fake Tensor](https://pytorch.org/torchdistx/latest/fake_tensor.html) and [Deferred Module Initialization](https://pytorch.org/torchdistx/latest/deferred_init.html) features.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76139
Approved by: https://github.com/bdhirsh
