Add prefix to examples in model_doc rst
# What does this PR do?

In my previous PR #11219, I was advised that the example should follow the syntax

```
>>> code_line_1
>>> code_line_2
result
```

I found some `model_doc` rst files that have code blocks without the prefix.
This PR intends to add `>>>` to those examples.

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [ ] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/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/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

Documentation: @sgugger

Hi @forest1988, thanks a lot for your PR! In parallel I've merged another PR that enforces proper indentation for those examples so now yours is conflicting. Could you rebase on master and solve the conflicts, or open a new PR from a fresh master, whichever is easier?
Thanks<|||||>Hi @sgugger,
Thank you for telling me that there is another PR merged and this PR has conflicts with it.
I've just rebased on master and solved the conflicts.

It seems there is a code quality problem, so I'll soon fix it.<|||||>`check_code_quality` shows the error message as below.
It seems something wrong happens during installing packages.

I'm sorry, but can you try to run circleci again?

```
#!/bin/bash -eo pipefail
pip install .[all,quality]
Defaulting to user installation because normal site-packages is not writeable
Processing /home/circleci/transformers
  Installing build dependencies ... - \ | / done
  Getting requirements to build wheel ... - done
    Preparing wheel metadata ... - \ done
Requirement already satisfied: filelock in /usr/local/lib/python3.6/site-packages (from transformers==4.6.0.dev0) (3.0.12)
Collecting dataclasses
  Using cached dataclasses-0.8-py3-none-any.whl (19 kB)
Requirement already satisfied: importlib-metadata in /usr/local/lib/python3.6/site-packages (from transformers==4.6.0.dev0) (1.7.0)
Collecting tqdm>=4.27
  Using cached tqdm-4.60.0-py2.py3-none-any.whl (75 kB)
Requirement already satisfied: requests in /usr/local/lib/python3.6/site-packages (from transformers==4.6.0.dev0) (2.25.1)
Collecting tokenizers<0.11,>=0.10.1
  Using cached tokenizers-0.10.2-cp36-cp36m-manylinux2010_x86_64.whl (3.3 MB)
Requirement already satisfied: packaging in /usr/local/lib/python3.6/site-packages (from transformers==4.6.0.dev0) (20.9)
Collecting regex!=2019.12.17
  Using cached regex-2021.4.4-cp36-cp36m-manylinux2014_x86_64.whl (722 kB)
Collecting numpy>=1.17
  Using cached numpy-1.19.5-cp36-cp36m-manylinux2010_x86_64.whl (14.8 MB)
Collecting sacremoses
  Using cached sacremoses-0.0.44-py3-none-any.whl
Requirement already satisfied: isort>=5.5.4 in /home/circleci/.local/lib/python3.6/site-packages (from transformers==4.6.0.dev0) (5.8.0)
Collecting black>=20.8b1
  Using cached black-20.8b1-py3-none-any.whl
Collecting flake8>=3.8.3
  Using cached flake8-3.9.0-py2.py3-none-any.whl (73 kB)
Collecting jaxlib>=0.1.59
  Using cached jaxlib-0.1.65-cp36-none-manylinux2010_x86_64.whl (44.7 MB)
Collecting soundfile
  Using cached SoundFile-0.10.3.post1-py2.py3-none-any.whl (21 kB)
Collecting tensorflow>=2.3
  Using cached tensorflow-2.4.1-cp36-cp36m-manylinux2010_x86_64.whl (394.3 MB)
Collecting torchaudio
  Using cached torchaudio-0.8.1-cp36-cp36m-manylinux1_x86_64.whl (1.9 MB)
Collecting Pillow
  Using cached Pillow-8.2.0-cp36-cp36m-manylinux1_x86_64.whl (3.0 MB)
Collecting keras2onnx
  Using cached keras2onnx-1.7.0-py3-none-any.whl (96 kB)
Collecting jax>=0.2.8
  Using cached jax-0.2.12-py3-none-any.whl
Collecting sentencepiece==0.1.91
  Using cached sentencepiece-0.1.91-cp36-cp36m-manylinux1_x86_64.whl (1.1 MB)
Collecting protobuf
  Using cached protobuf-3.15.8-cp36-cp36m-manylinux1_x86_64.whl (1.0 MB)
Collecting flax>=0.3.2
  Using cached flax-0.3.3-py3-none-any.whl (179 kB)
Collecting torch>=1.0

Received "killed" signal
```<|||||>Thanks, I applied all suggestions!

I'm sorry, I misunderstood the meaning of the following two that appear in https://huggingface.co/transformers/_sources/quicktour.rst.txt, and assumed that double # were required for comments in the Transformers documentation.
`## PYTORCH CODE` ` ## TENSORFLOW CODE`

If I'm not mistaken in my current understanding, these are special codes to switch between PyTorch and TensorFlow versions, right?
<|||||>Yes those are special markers, which is why they have the double #, for regular comments, we use just one #