Fix Torch.hub + Integration test
- Torch.hub doesn't use pip-installed versions of modules, but uses a [custom importer instead](https://github.com/pytorch/pytorch/blob/master/torch/hub.py#L70-L83) (it imports `hubconf.py`) which means that:
  - all imports from hubconf.py refer to src.transformers instead of transformers
  - all imports inside the lib's code **must** be relative, i.e. shouldn't assume that the transformers module is installed (it's not)
- Added a GitHub action workflow to ensure that `hub.list` and `hub.help` always work.
(Hmm, GitHub was failing earlier today, and now it seems to have posted my comment multiple times. Sorry about that.)<|||||>See cbbb3c43c55d2d93a156fc80bd12f31ecbac8520