[WIP][Doctests] Fix rst -> mdx
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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In this PR, the already doc tests are enabled. The changes to `quickstart.mdx` show the necessary changes to make the doc tests work


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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
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- gpt2: @patrickvonplaten, @LysandreJik
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Library:

- benchmarks: @patrickvonplaten
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- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
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- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15329). All of your documentation changes will be reflected on that endpoint.

@LysandreJik @sgugger, It'd be great if you could take a first look here.

There are essentially two things that I'd like to verify/discuss here:
1. The seperation line `===PT-TF-SPLIT===` in Python code ```python is interpreted as an output which then leads the doctest to fail with:
```
Expected: ===PT-TF-SPLIT===
Got: Nothing
```
In this PR I replaced `===PT-TF-SPLIT===` with `>>> # ===PT-TF-SPLIT===` so that the split is considered as a comment by doctests and they pass. I don't really see another way here. @sgugger - could we adapt the doc-builder to correctly parse `>>> # ===PT-TF-SPLIT===` instead of `===PT-TF-SPLIT===`?

2. The second problem is that if the code snippet ends with ``` directly after the last python command, the doctests also interpret this as an expected output. This is why I added a newline before every ``` in the end which is not really a good idea IMO. I think it might make sense to just a quick preprocessing of all files that we run the doc tests on to add an empty space before the final ``` -> and then run the doc tests. Think this is better than adding a new line before every ``` which also doesn't go well with @sgugger's style_doc.py file. Also see: https://stackoverflow.com/questions/61163110/python-doctests-embedded-in-readme-md-expected-got-nothing here

Closing this one as it's more or less a duplicate of the one we merged last week and requires the `PT-TF` switch to be solved before. Will open a new one then :-)