Add codemod for TorchNonPublicAliasVisitor

[kit1980]

This is a continuation of #33

We want to update non-public function calls to public aliases, possibly adding or modifying imports in the process.
The call sites needs to be updated to the same hierarchical level as in the existent code (as much as possible):
collate.default_collate() -> dataloader.default_collate(), but torch.utils.data._utils.collate.default_convert() -> torch.utils.data.dataloader.default_convert().

See the tests for different cases.

[seemethere]

Is my understanding correct here that this basically searches recursively down the node tree for a function with the same name that exists in a public package and then attempts to replace the private function with that new public function?

Might also be good to add a description to the PR / a comment to the code as well to explain some of this logic since it's somewhat hard to grok quickly from reading the code.

[kit1980]

Is my understanding correct here that this basically searches recursively down the node tree for a function with the same name that exists in a public package and then attempts to replace the private function with that new public function?

There is no recursion.
There is a mapping "non-public" -> "public" defined here https://github.com/pytorch-labs/torchfix/blob/main/torchfix/visitors/nonpublic/__init__.py#L21

A somewhat tricky part is that for example _utils.collate.default_collate() needs to be updated to dataloader.default_collate() and then a new import needs to be added. And in other cases imports need to be modified, and maybe call site needed to be leave as is: see the tests.