Remove hard-coded pad token id in distilbert and albert
Author: monologg
Pull request: yes

As the config adds `pad_token_id` attribute, `padding_idx` set the value of `config.pad_token_id` in BertEmbedding. ( PR #3793 )

But it seems that not only the config of `Bert`, but also that of `DistilBert` and `Albert` has `pad_token_id`. ([Distilbert config](https://s3.amazonaws.com/models.huggingface.co/bert/distilbert-base-uncased-config.json), [Albert config](https://s3.amazonaws.com/models.huggingface.co/bert/albert-base-v1-config.json))

But in Embedding class of Distilbert and Albert, it seems that `padding_idx` is still hard-coded. So I've fixed those parts.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3965?src=pr&el=h1) Report
> Merging [#3965](https://codecov.io/gh/huggingface/transformers/pull/3965?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/73d6a2f9019960c327f19689c1d9a6c0fba31d86&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3965/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3965?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3965      +/-   ##
==========================================
- Coverage   78.45%   78.44%   -0.02%     
==========================================
  Files         111      111              
  Lines       18518    18518              
==========================================
- Hits        14528    14526       -2     
- Misses       3990     3992       +2     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3965?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/3965/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hbGJlcnQucHk=) | `75.31% <100.00%> (ø)` | |
| [src/transformers/modeling\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/3965/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19kaXN0aWxiZXJ0LnB5) | `98.15% <100.00%> (ø)` | |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3965/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `68.49% <0.00%> (-0.37%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3965/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.94% <0.00%> (-0.13%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3965?src=pr&el=footer). Last update [73d6a2f...9d92a30](https://codecov.io/gh/huggingface/transformers/pull/3965?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


LGTM

@LysandreJik @VictorSanh 

Hi:) Can you please check this PR? This one makes issue on Korean BERT. (which use `pad_token_id=1` and `unk_token_id=0`)

I hope this PR will be applied on the next version of transformers library:)

lgtm!

@julien-c 

Can you merge this PR? Thank you so much:)