Fix `AttributeError` from `PreTrainedTokenizerFast.decoder`
# What does this PR do?

Calling the `decoder` attribute for the T5 fast tokenizer raises an error because it tries accessing the non-existent `_tokenizer` field of `tokenizers.Tokenizer`. In `PreTrainedTokenizerFast.decoder`, `self._tokenizer._tokenizer.decoder` is executed, which contains a probably unintentional repeat of `_tokenizer`.

Before:

```python
>>> from transformers import AutoTokenizer
>>> tokenizer = AutoTokenizer.from_pretrained('t5-base')
>>> type(tokenizer)
<class 'transformers.models.t5.tokenization_t5_fast.T5TokenizerFast'>
>>> tokenizer.decoder
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "transformers/src/transformers/tokenization_utils_fast.py", line 183, in decoder
    return self._tokenizer._tokenizer.decoder
AttributeError: 'tokenizers.Tokenizer' object has no attribute '_tokenizer'
```

After:

```python
>>> from transformers import AutoTokenizer
>>> tokenizer = AutoTokenizer.from_pretrained('t5-base')
>>> type(tokenizer)
<class 'transformers.models.t5.tokenization_t5_fast.T5TokenizerFast'>
>>> tokenizer.decoder
<tokenizers.decoders.Metaspace object at 0x190c817b0>
```

I'm not sure how this hasn't been caught before. The line has not been touched since June 2020, and the `decoder` property is presumably being used somewhere. It's hard to tell if it has any usages in the repository because PyCharm finds a large number of unrelated fields named `decoder` when I search.

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests), Pull Request section?
- [x] 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.
- [x] 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?

tokenizers: @n1t0, @LysandreJik
Pinging @n1t0, @LysandreJik.