Fix broken FX tests (#83187)
Resolves [breakages](https://github.com/pytorch/pytorch/runs/7762125339?check_suite_focus=true) observed at #82560
Context:
The current FX tests assume that every public method under `torchvision.models` is a model builder method. To get a list of those methods, they query the `__dict__` attribute of the module. Unfortunately this assumption is not true and the tests already contain some workarounds to filter some methods. A better approach would be to query TorchVision for all of its available models under a specific module. This is exactly what the new Registration API can help us do and that's what we use in this PR.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/83187
Approved by: https://github.com/ezyang
