fix retribert's `test_torch_encode_plus_sent_to_model`
Author: SaulLu
Pull request: yes

# What does this PR do?

This PR proposes a fix for the slow test `test_torch_encode_plus_sent_to_model` for RetriBert that failed in the CI daily that ran yesterday.

The error is due to the fact that RetriBert is not a classical model in the sense that the input can be fed to one or two encoders of the model. As a result `get_input_embeddings` is an unimplemented method and the forward method expects more arguments in input than what is outputed by the tokenizer.

I have therefore overridden the common test with a specific test for RetriBert that reflects these two specificities, which I will highlight them in a comment below.


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed

## Local test

I've tested it by running:
```bash
RUN_SLOW=yes pytest tests/models/retribert/ -k test_torch_encode_plus_sent_to_model
```
output:
```
=========================================================================================== test session starts ============================================================================================
platform linux -- Python 3.9.12, pytest-7.1.1, pluggy-1.0.0
rootdir: /home/lucile_huggingface_co/repos/transformers, configfile: setup.cfg
plugins: dash-2.3.1, hypothesis-6.41.0, timeout-2.1.0, forked-1.4.0, xdist-2.5.0
collected 98 items / 97 deselected / 1 selected                                                                                                                                                            

tests/models/retribert/test_tokenization_retribert.py .                                                                                                                                              [100%]

============================================================================== 1 passed, 97 deselected, 10 warnings in 5.38s ===============================================================================
```


Comments:
_The documentation is not available anymore as the PR was closed or merged._

@ydshieh , this is a good question (which I also asked myself, I should have shared my thoughts on it). 

Actually `self.bert_doc` is set to None in the model that is loaded. So, we can't test anything on `self.bert_doc` and `embed_answers` will be identical to `embed_questions` (and I remembered that we're testing the tokenizer not the model). But I follow what you think is best, i.e. if you think that it would be better to test the other 2 cases as a precaution if the checkpoint ever changes :hugs: 

> `self.bert_doc` is set to None

In this case, I think we can just keep what you have done so far in this PR, no need to change 😄 . Thanks for the info.

@SaulLu Guess we can merge this PR now :-) ?