Add X-CLIP
# What does this PR do?

This PR adds [X-CLIP](https://github.com/microsoft/VideoX/tree/master/X-CLIP), which is a minimal extension of CLIP for video-language pre-training.

To do:

- [x] upload all checkpoints to the hub, as part of the `microsoft` organization


Many tests fail due to the following error:

> ModuleNotFoundError: No module named 'transformers.models.xclip'

This is probably because I first called the model folder "xclip", which is now called "x_clip". Still, wondering why it keeps looking for the module models.clip. If anyone has any pointers, that would be greatly appreciated.<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>@sgugger thanks a lot, that solved the issue. There seems to be another (small) issue with run_tests_hub:
```
==================================== ERRORS ====================================
_______________ ERROR collecting tests/utils/test_file_utils.py ________________
tests/utils/test_file_utils.py:26: in <module>
    from transformers import *  # noqa F406
src/transformers/utils/import_utils.py:1021: in __getattr__
    value = getattr(module, name)
src/transformers/utils/import_utils.py:1023: in __getattr__
    raise AttributeError(f"module {self.__name__} has no attribute {name}")
E   AttributeError: module transformers.models.clip has no attribute CLIPProcessor
```
Running `RUN_SLOW=yes pytest tests/utils/test_file_utils.py` passes locally for me.<|||||>That would be because you moved `CLIPProcessor` in the non-vision dependent objects in the main init (and rightly so) but did not do the same for the `models/clip/__init__.py`.<|||||>@sgugger and @alaradirik - the PR is ready for merge. Kindly asking for your approval :)