Enable 'from transformers import AlbertMLMHead'
Discussed at https://github.com/huggingface/transformers/issues/2894

I'm writing a custom pretraining script that incorporates both the masked language modeling (MLM) and sentence order prediction (SOP) objectives. I'm able to use the TFAlbertForMaskedLM model for the MLM objective, but need access to the last_hidden_state to write my SOP objective. I can do this if I have a raw TFAlbertModel and write my own MLM objective, but would prefer to just create my own model from pre-existing modularized components.

I know that there's a lot of care taken in API design, and the team may have explicitly decided against this. But if it is an option, it would make the transformers repo much more extensible for research.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2895?src=pr&el=h1) Report
> Merging [#2895](https://codecov.io/gh/huggingface/transformers/pull/2895?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/2ae98336d17fceea7506af9880b862b6252a38f6?src=pr&el=desc) will **decrease** coverage by `1.07%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2895/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2895?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2895      +/-   ##
==========================================
- Coverage   75.06%   73.98%   -1.08%     
==========================================
  Files          94       94              
  Lines       15288    15288              
==========================================
- Hits        11476    11311     -165     
- Misses       3812     3977     +165
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2895?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/2895/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9fX2luaXRfXy5weQ==) | `98.87% <ø> (ø)` | :arrow_up: |
| [src/transformers/modeling\_tf\_pytorch\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2895/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9weXRvcmNoX3V0aWxzLnB5) | `8.72% <0%> (-81.21%)` | :arrow_down: |
| [src/transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/2895/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yb2JlcnRhLnB5) | `85.91% <0%> (-9.86%)` | :arrow_down: |
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/2895/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `70.86% <0%> (-2.3%)` | :arrow_down: |
| [src/transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/2895/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19jdHJsLnB5) | `92.07% <0%> (-2.21%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/2895/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `80.2% <0%> (-1.35%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2895?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2895?src=pr&el=footer). Last update [2ae9833...014ad24](https://codecov.io/gh/huggingface/transformers/pull/2895?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>(I removed myself because GitHub suggests me as a reviewer on every PR because of a refactoring, and I can't review every PR, not because this is a bad idea. The PR is most likely good.)