CLI: add stricter automatic checks to `pt-to-tf`
# What does this PR do?

Last week I introduced the `pt-to-tf` CLI (https://github.com/huggingface/transformers/pull/17497), enabling automatic weight conversion followed by PR opening.

This PR makes four changes related to that CLI:
1. Uses the appropriate model class to load the model, to ensure the head's weights also get converted;
2. Adds much stricter checks -- ALL model outputs (with `output_hidden_states=True`) are verified;
3. Adds a flag to create new TF weights, even if they already exist;
4. Updates the docker file for the scheduled tests to install `git lfs` -- I did it for the circleci workflows in the original PR, but forgot to do it for the scheduled tests.

🚨 This also means I will double-check previously open Hub PRs (about 10), to confirm that the model head is present in the TF weights (I suspect it isn't in some cases 😢 ) and that the outputs pass the stricter tests.

For context, if the conversion fails because of a difference in the model outputs, we get a message like this one:
<img width="1013" alt="Screenshot 2022-06-07 at 17 00 12" src="https://user-images.githubusercontent.com/12240844/172427441-e1fd63e4-7887-47bf-8827-9209bc1df78a.png">
_The documentation is not available anymore as the PR was closed or merged._<|||||>LGTM, just a few nits if they make sense.
It's indeed very important regarding the `head`, great catch, @gante !
