Add classifier dropout in ALBERT
As mentioned in the original [paper](https://arxiv.org/pdf/1909.11942.pdf), they separated the dropout rates of the transformer cells and the classifier, moreover, in V2 the dropouts are 0 (expect for the classifier, again).

Current implementation does not supports this and models are not training well (can't reproduce results of GLUE benchmark using V2 models). I manually updated these values and got V2 models converging.

This issue was raised in #2337 and also mentioned in https://github.com/google-research/ALBERT/issues/23

I added a separate parameter in the config file and update the sequence classification head.

Please also update the configuration of ALBERT V2 models ([base](https://huggingface.co/albert-base-v2), [large](https://huggingface.co/albert-large-v2), [xlarge](https://huggingface.co/albert-xlarge-v2)) in your repository.
More specifically, the configuration of the **attention and hidden dropout rates** of ALBERT V2 models in your repository as well (see as in https://tfhub.dev/google/albert_base/3, https://tfhub.dev/google/albert_large/3, https://tfhub.dev/google/albert_xlarge/3 and https://tfhub.dev/google/albert_xxlarge/3)

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2679?src=pr&el=h1) Report
> Merging [#2679](https://codecov.io/gh/huggingface/transformers/pull/2679?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/83446a88d902661fab12bf8c37a1aa2845cdca5f?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/2679/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2679?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2679      +/-   ##
==========================================
+ Coverage   74.59%   74.59%   +<.01%     
==========================================
  Files          89       89              
  Lines       14971    14972       +1     
==========================================
+ Hits        11168    11169       +1     
  Misses       3803     3803
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2679?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/2679/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hbGJlcnQucHk=) | `79.14% <100%> (ø)` | :arrow_up: |
| [src/transformers/configuration\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/2679/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2FsYmVydC5weQ==) | `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/2679?src=pr&el=footer). Last update [83446a8...12c7809](https://codecov.io/gh/huggingface/transformers/pull/2679?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>That's fantastic, thank you for taking the time to do this @peteriz !<|||||>The configuration files were updated. The type of GELU activation function used was also changed to "gelu_new", which is the appropriate activation function that is used in the google-research repository.

[Original gelu](https://github.com/google-research/ALBERT/blob/e8f8339b003cf2ddbb5ee9fc34a32651b33dd64e/modeling.py#L296-L309)

[Our gelu new](https://github.com/huggingface/transformers/blob/master/src/transformers/modeling_bert.py#L138-L142)