Update to include example of LM
Author: iliaschalkidis
Pull request: yes

The model files have been updated in order to include the classification layers, based on https://github.com/huggingface/transformers/issues/2901, and now can be also used as a LM.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2903?src=pr&el=h1) Report
> Merging [#2903](https://codecov.io/gh/huggingface/transformers/pull/2903?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/20fc18fbda3669c2f4a3510e0705b2acd54bff07?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/2903/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2903?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2903      +/-   ##
==========================================
- Coverage      75%   73.92%   -1.08%     
==========================================
  Files          94       94              
  Lines       15288    15288              
==========================================
- Hits        11467    11302     -165     
- Misses       3821     3986     +165
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2903?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_pytorch\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2903/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/2903/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/2903/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/2903/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/2903/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/2903?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/2903?src=pr&el=footer). Last update [20fc18f...afa57d9](https://codecov.io/gh/huggingface/transformers/pull/2903?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


[Looks good!](https://huggingface.co/nlpaueb/bert-base-greek-uncased-v1)

@julien-c Could you perhaps shed some light on how the TF checkpoints should be uploaded? @iliaschalkidis asked about it here https://github.com/huggingface/transformers/issues/2901#issuecomment-588163505

In the past for another project, this (`from_pt=True`) did the dirty trick for me:

```python
bert = BERT.from_pretrained(model_path+'pytorch_model.bin',
                                               from_pt=True,
                                               config=BertConfig().from_pretrained(model_path+'config.json'))
```
but I definitely not recommend this...

I have already uploaded the TF checkpoint files (`model_ckpt.data-00000-of-00001`, `model_ckpt.index`, `model_ckpt.meta`) in model's folder, so please feel free to troubleshoot.

see https://github.com/huggingface/transformers/issues/2901#issuecomment-591710959