Add PRETRAINED_INIT_CONFIGURATION to DistilBERT tokenizer
The DistilBERT tokenizer does not make use of `PRETRAINED_INIT_CONFIGURATION`, instead loading BERT's.

This PR fixes this, fixing the issue detailed in #2423.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2469?src=pr&el=h1) Report
> Merging [#2469](https://codecov.io/gh/huggingface/transformers/pull/2469?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/f599623a99b808e3d5926d89cd13237457b9eeba?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2469/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2469?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2469      +/-   ##
==========================================
+ Coverage   73.23%   73.24%   +<.01%     
==========================================
  Files          87       87              
  Lines       15003    15005       +2     
==========================================
+ Hits        10988    10990       +2     
  Misses       4015     4015
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2469?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/2469/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fZGlzdGlsYmVydC5weQ==) | `100% <100%> (ø)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2469?src=pr&el=footer). Last update [f599623...89df3b4](https://codecov.io/gh/huggingface/transformers/pull/2469?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
