Add script to convert tf2.x checkpoint to PyTorch
The script converts the newer TF2.x checkpoints (as published on their [official GitHub](https://github.com/tensorflow/models/tree/master/official/nlp/bert) to Pytorch. The [existing script](https://github.com/huggingface/transformers/blob/master/src/transformers/convert_bert_original_tf_checkpoint_to_pytorch.py) only works with checkpoints from the [original BERT repository](https://github.com/google-research/bert) which uses TF 1.4.

The script currently only converts the encoder part (but no MLM/NSP heads). The official checkpoints published by the tensorflow team unfortunately also don't contain these heads. I have written a script which takes care of these, but it does add a fair bit of complexity.

I have tested on my side by comparing all model weights with the official Huggingface version:

```python
from transformers import BertModel
import torch

def validate_model(bert_original, bert_converted):
    assert bert_original.num_parameters() == bert_converted.num_parameters()
    assert len(bert_original.state_dict()) == len(bert_converted.state_dict())
    for (layer_original, value_original), (layer_converted, value_converted) in zip(bert_original.state_dict().items(), bert_converted.state_dict().items()):
        assert layer_original == layer_converted
        if not torch.eq(value_original, value_converted).all():
            raise ValueError(f'Incorrect weights for {layer_original}')
    print('Success! Both models are identical!')

if __name__ == "__main__":
    validate_against = 'bert-base-uncased'
    path_to_converted_model = './converted_bert_base_uncased'
    bert_converted = BertModel.from_pretrained(path_to_converted_model)
    bert_original = BertModel.from_pretrained(validate_against)
    validate_model(bert_original, bert_converted)
```

I'm happy to write some tests for this if needed (and if possible) or have any other input.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5791?src=pr&el=h1) Report
> Merging [#5791](https://codecov.io/gh/huggingface/transformers/pull/5791?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/3b924fabeef717be8399f1888280c29c69e9ab00&el=desc) will **decrease** coverage by `0.08%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5791/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5791?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5791      +/-   ##
==========================================
- Coverage   78.13%   78.05%   -0.09%     
==========================================
  Files         146      146              
  Lines       26047    26047              
==========================================
- Hits        20352    20330      -22     
- Misses       5695     5717      +22     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5791?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/5791/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `64.90% <0.00%> (-33.90%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5791/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `85.96% <0.00%> (-0.26%)` | :arrow_down: |
| [src/transformers/generation\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5791/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3V0aWxzLnB5) | `97.11% <0.00%> (+0.28%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/5791/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `93.36% <0.00%> (+49.37%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5791?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/5791?src=pr&el=footer). Last update [3b924fa...378b034](https://codecov.io/gh/huggingface/transformers/pull/5791?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Other than that, LGTM! Thanks for your work!<|||||>Great, just renamed it - let me know if anything else should be changed!