add autocast keys to pybind11 DispatchKey object (#90821)
Summary:
This is useful for debugging what autocast is doing when
it's running on top of torchdynamo, without this the Python dispatch
key for autocast prints as `???`.
Test Plan:
```
import torch
dir(torch._C.DispatchKey)
// the autocast keys show up now
```
Reviewers:
Subscribers:
Tasks:
Tags:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90821
Approved by: https://github.com/ezyang
