Tokenizers docs: Specify which class contains `__call__` method
Author: xhluca
Pull request: yes

Currently, the docs specify the following:

> BatchEncoding holds the output of the tokenizer’s encoding methods (__call__, encode_plus and batch_encode_plus) and is derived from a Python dictionary. 

It's not clear what tokenizer class this is referring to. Moreover, the [main `tokenizer`](https://huggingface.co/transformers/main_classes/tokenizer.html) page does not have any documentation for `__call__`; instead it is found in `PreTrainedTokenizerBase`. 

The proposed change in this PR will make it clear where the user can found documentation about the `__call__` function, which is very widely used now.

Comments:
@n1t0 This is one instance of `__call__`, but maybe it would be beneficial if all `__call__` in the docs links to [this docstring](https://huggingface.co/transformers/internal/tokenization_utils.html#transformers.tokenization_utils_base.PreTrainedTokenizerBase.__call__)? Let me know your thoughts!

Cool, I think this is a welcome change! cc @sgugger

Could you run the style utilities to fix the code quality issues? You can do so by running this from the root of the repository:
```
pip install -e ".[quality]"
make fixup
```

Hi @LysandreJik I'm having trouble with that command. Got the following issue:

```
(venv) xhlu@XHL-Desktop:~/dev/transformers$ make fixup
No library .py files were modified
python utils/custom_init_isort.py
python utils/style_doc.py src/transformers docs/source --max_len 119
running deps_table_update
updating src/transformers/dependency_versions_table.py
python utils/check_copies.py
python utils/check_table.py
None of PyTorch, TensorFlow >= 2.0, or Flax have been found. Models won't be available and only tokenizers, configuration and file/data utilities can be used.
python utils/check_dummies.py
python utils/check_repo.py
None of PyTorch, TensorFlow >= 2.0, or Flax have been found. Models won't be available and only tokenizers, configuration and file/data utilities can be used.
None of PyTorch, TensorFlow >= 2.0, or Flax have been found. Models won't be available and only tokenizers, configuration and file/data utilities can be used.
Checking all models are included.
Checking all models are public.
Checking all models are properly tested.
Checking all objects are properly documented.
Checking all models are in at least one auto class.
utils/check_repo.py:400: UserWarning: Full quality checks require all backends to be installed (with `pip install -e .[dev]` in the Transformers repo, the following are missing: PyTorch, TensorFlow, Flax. While it's probably fine as long as you didn't make any change in one of those backends modeling files, you should probably execute the command above to be on the safe side.
  warnings.warn(
python utils/check_inits.py
python utils/tests_fetcher.py --sanity_check
Traceback (most recent call last):
  File "utils/tests_fetcher.py", line 23, in <module>
    from git import Repo
ModuleNotFoundError: No module named 'git'
make: *** [Makefile:42: repo-consistency] Error 1
```

Not sure what's causing. I do have `git` installed, but this seems like it's trying to import some `git` module

I believe the package to install is `gitpython`, we should add this to the setup 

If you rebase on `master` and re-run the commands above it should work!

@LysandreJik thanks. I applied the `make fixup` and commited the change.

Thanks again for your PR!

Glad it was helpful :)