Add type annotations to nnapi (#48142)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/48141
~Mypy is complaining about a missing arg in a function call.~
```bash
torch/backends/_nnapi/serializer.py:806: error: Too few arguments for "_do_add_binary" [call-arg]
Found 1 error in 1 file (checked 1140 source files)
```
https://github.com/pytorch/pytorch/blob/9392137dbe4fa6e146c5312636c1fb76b9889ece/torch/backends/_nnapi/serializer.py#L804-L806
~dreiss, would you mind take a look when you have some cycles to spare and see what would be the appropriated value for `fuse_code` here? Thanks :)~
Edit: https://github.com/pytorch/pytorch/issues/48925 got merged a couple of days ago. The blocking part is now unblocked, and I just pushed the changes to make mypy happy again. This PR is ready for review.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48142
Reviewed By: ezyang
Differential Revision: D28006249
Pulled By: walterddr
fbshipit-source-id: 5e43eeba7143512a549efaad31541f86718add7c
