Added AutocastCPU string (#70013)
Summary:
Description:
- Added "AutocastCPU" string repr into `toString` method
Before
```
std::cout << c10::DispatchKey::AutocastCPU;
> UNKNOWN_TENSOR_TYPE_ID
```
and now:
```
std::cout << c10::DispatchKey::AutocastCPU;
> AutocastCPU
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70013
Reviewed By: ejguan
Differential Revision: D33550777
Pulled By: bdhirsh
fbshipit-source-id: b31e15e6d52fc1768af085e428328117d588f283
