Raise a TF-specific error when importing Torch classes
We've had a couple of reports from users that the class naming is confusing - they import `AutoModel` but don't realize that it's a PyTorch-only class, and the error message doesn't give them any guidance except to tell them to install PyTorch.

This PR adds a check to `requires_backends`, so that if the user tries to import a class that requires PyTorch when they don't have it installed, but they do have TF installed instead, it will raise an error that explains the situation and directs them to the TF classes.

Fixes #18220.
_The documentation is not available anymore as the PR was closed or merged._<|||||>@LysandreJik I'm not sure! I think it's more obvious that "TFAutoModel" is TF-specific, and it's only the PyTorch classes that don't specify the framework in their names. But I guess it's okay to add the inverse error too, one sec!<|||||>@Rocketknight1 NICE! -- Thanks for implementing this.  You rock!