Add TFCLIPModel
# What does this PR do?


Add `TFCLIPModel`, along with `TFCLIPTextModel` and `TFCLIPVisionModel`.

(and fixed doc examples in `modeling_clip.py`)

## Who can review?
@Rocketknight1 @patrickvonplaten @NielsRogge @sgugger @LysandreJik 
Finally finalized this PR after working on something else. It is ready for review :)<|||||>@patil-suraj You know CLIP much better than me, but feel free to assign me as a reviewer too if you want me to double-check the TF code!<|||||>Thanks, @Rocketknight1 . I will re-work the initialization part (forgot that PT model codes have `_init_weights`).
Could you check my comment about your question on `tf.tile/tf.broadcast_to`. I have a question there.<|||||>I addressed all the reviews, in particular the weight initialization.

In PT version, there is 

```
if isinstance(module, CLIPTextEmbeddings):
    module.token_embedding.weight.data.normal_(mean=0.0, std=factor * 0.02)
    module.position_embedding.weight.data.normal_(mean=0.0, std=factor * 0.02)
```

I am not sure why `0.02` is used instead of `config.initializer_range`. cc @patil-suraj <|||||>Should this be merged @Rocketknight1 @patil-suraj? :smiley: <|||||>> Should this be merged @Rocketknight1 @patil-suraj? 😃

There is (new) conflicts I just saw. I will resolve later today or tomorrow.<|||||>okay for merge on my side (once the conflicts are resolved) !<|||||>Merging will require:
- a rebase on master
- converting all docstrings to Markdown.

You can install our `doc-builder` utility to do this:
```
pip install git+https://github.com/huggingface/doc-builder
```

Then run from your branch in the Transformers repo
```
doc-builder convert src/transformers/models/clip/modeling_tf_clip.py
```
to do this in the newly added modeling_tf_clip file.

Note that clip.rst has disappeared and you will need to add your model to clip.mdx instead.

Let us know if you need any help and please ping for review when you're done!<|||||>@patil-suraj @Rocketknight1 @sgugger I rebased on master and fixed errors. The last failed test is irrelevant I think.
Ready to be merged when you have some time :)

Thank you for reviewing this PR!<|||||>@sgugger I'm happy at this point!