[CI] use correct deps for torchhub
As a follow up to https://github.com/huggingface/transformers/pull/9550, here is a clean solution that requires only one source  (`setup.py`) to edit for dependencies and groups of thereof.

The PR 
1. defines a new dependency group for `torchhub` in `setup.py`
2. installs the exact dependencies of that group inside .github workflow
3. uninstalls  `transformers` since Sylvain said it shouldn't be there, but it had to be installed to get the deps easily.

Of course, it'll need to be merged first for:
```
pip install -e git+https://github.com/huggingface/transformers.git#egg=transformers[torchhub]
```
to work, since it's not there now... meanwhile you can test it from this branch:
```
pip install -e git+https://github.com/stas00/transformers.git@torchhub-deps#egg=transformers[torchhub]
```

-------------

Alternatively to:
```
pip install -e git+https://github.com/huggingface/transformers.git#egg=transformers[torchhub]
pip uninstall -y transformers
```
we can do:
```
git clone https://github.com/huggingface/transformers
cd transformers
pip install -e .[torchhub]
pip uninstall -y transformers
```
for a few fractions of seconds faster.

----------------

Yet, another approach it to extend `setup.py` with what I created here a few year back:

https://github.com/fastai/fastai1/blob/a8327427ad5137c4899a1b4f74745193c9ea5be3/setup.py#L11-L22

This then:
```
python setup.py -q deps --dep-groups=torchhub
```
would dump the dependencies just for the specified extra groups, which can then be fed to `pip install`, so there will be no need to install the main package. Literally, the above command would just dump `extras["torchhub"]` in this case.

--------------

Finally, we could make `src/transformers/dependency_versions_table.py` contain the full dependency groups as well, and again then it'll just be needed to get one's hands on that file to extract groups of dependencies, e.g.:

```
wget https://raw.githubusercontent.com/huggingface/transformers/master/src/transformers/dependency_versions_table.py
python -c 'import sys; from dependency_versions_table import dep_group; print(dep_group[sys.argv[1]])' torchhub
```
this is hypothetical since we don't currently have `dep_group` dict in `dependency_versions_table.py`.

@sgugger, @LysandreJik  
Is there a way to leverage this to also update the dependencies in [`hubconf.py`](https://github.com/huggingface/transformers/blob/master/hubconf.py)?<|||||>From what I understand, you need to install the dependencies by hand before, so what would it add to have this in hubconf? From what I gathered this list of "dependencies" is just there to be dynamically imported when executing the code to import the model, but it's not doing anything for the install.<|||||>> From what I understand, you need to install the dependencies by hand before, so what would it add to have this in hubconf? From what I gathered this list of "dependencies" is just there to be dynamically imported when executing the code to import the model, but it's not doing anything for the install.

What Sylvain said.

Moreover we validated that yesterday, since trying to add specific versions in `hubconf.py` made no difference. So that var should have been called "imports" to be more precise.
