Add missing arguments for BertWordPieceTokenizer
Author: monologg
Pull request: yes

Hi:)

It seems that in `__init__` of `BertTokenizerFast`, arguments of `pad_token` and `mask_token` of `BertWordPieceTokenizer` are missing.

I've added them in this commit. If these missed arguments are intended, please let me know:)

Thank you!

```python
class BertTokenizerFast(PreTrainedTokenizerFast):
    def __init__(
        self,
        vocab_file,
        do_lower_case=True,
        unk_token="[UNK]",
        sep_token="[SEP]",
        pad_token="[PAD]",
        cls_token="[CLS]",
        mask_token="[MASK]",
        clean_text=True,
        tokenize_chinese_chars=True,
        strip_accents=None,
        wordpieces_prefix="##",
        **kwargs
    ):
        super().__init__(
            BertWordPieceTokenizer(
                vocab_file=vocab_file,
                unk_token=unk_token,
                sep_token=sep_token,
                cls_token=cls_token,
                clean_text=clean_text,
                handle_chinese_chars=tokenize_chinese_chars,
                strip_accents=strip_accents,
                lowercase=do_lower_case,
                wordpieces_prefix=wordpieces_prefix,
            ),
            unk_token=unk_token,
            sep_token=sep_token,
            pad_token=pad_token,
            cls_token=cls_token,
            mask_token=mask_token,
            **kwargs,
        )
```

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5810?src=pr&el=h1) Report
> Merging [#5810](https://codecov.io/gh/huggingface/transformers/pull/5810?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/89a78be51f1c2afd263da66ec76d3297432f0c2a&el=desc) will **decrease** coverage by `0.88%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5810/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5810?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5810      +/-   ##
==========================================
- Coverage   78.19%   77.30%   -0.89%     
==========================================
  Files         146      146              
  Lines       26047    26047              
==========================================
- Hits        20367    20136     -231     
- Misses       5680     5911     +231     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5810?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/5810/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmVydC5weQ==) | `91.32% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/5810/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9tb2JpbGViZXJ0LnB5) | `23.38% <0.00%> (-73.39%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/5810/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `64.90% <0.00%> (-33.90%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5810/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.46% <0.00%> (+0.25%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5810/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `81.49% <0.00%> (+0.29%)` | :arrow_up: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5810/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `82.31% <0.00%> (+1.28%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/5810/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `93.36% <0.00%> (+49.37%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5810/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9vcGVuYWkucHk=) | `95.18% <0.00%> (+74.91%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5810?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5810?src=pr&el=footer). Last update [89a78be...d7b3627](https://codecov.io/gh/huggingface/transformers/pull/5810?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


@LysandreJik 

Can you merge this PR? Thank you:)