Need suggestion on contributing TFDPR
# 🌟 New model addition

## Model description
Hi, I would love to try contributing TFDPR . This is the first time to me, so I need some suggestions.
I have followed @sshleifer 's [great PR on TFBart model](https://github.com/huggingface/transformers/commit/829842159efeb1f920cbbb1daf5ad67e0114d0b9) on 4 files :` __init__.py , convert_pytorch_checkpoint_to_tf2.py , utils/dummy_tf_objects.py` and (newly created) `modeling_tf_dpr.py `

Now the TF model works properly and can load Pytorch's weights successfully the same output as Pytorch's counterparts **except** small random noise (1e-5) which I suspect of some dtypes different , but I could not find the cause. 

I guess I need to add document on  docs/source/model_doc/dpr.rst , and that's all ? 
**My question is do I need to change / fix any other files ? and/or do I need to do some other thing before making PR ?**

<!-- Important information -->
To resolve TF vs. Pytorch naming issues, there's one change regarding `TFBertModel` vs. `TFBertMainLayer` as [discussed here](https://discuss.huggingface.co/t/solved-issue-on-translating-dpr-to-tfdpr-on-loading-pytorch-weights-to-tf-model/1764) .
Thanks to @sshleifer for his help to solve the issue.

## Open source status

* [X] the model implementation is available: (give details)
You can see all the modified codes with test run at : 
https://colab.research.google.com/drive/1lU4fx7zkr-Y3CXa3wmHIY8yJhKdiN3DI?usp=sharing
(to easily navigate the changes, please “find on page” for e.g. `TFDPRContextEncoder` )

* [X] the model weights are available: (give details)
At the moment, I use existing Pytorch weights, but will upload TF weights too.

* [X] who are the authors: (mention them, if possible by @gh-username)
@ratthachat 
Hello! Thanks for offering to contribute the TF implementation of the DPR model! Something that may help you is to open a PR very early on, even if you have a lot of questions. This way we can help provide pointers, and we can guide you in the right direction. 

Another aspect that may be of tremendous help, would be to follow the checklist when adding a new model. It is available [here](https://github.com/huggingface/transformers/tree/master/templates/adding_a_new_model). If you open a PR, we recommend to put this checklist in the description so that everybody can follow better.

Let me know if I can help further.<|||||>@LysandreJik Thanks for your suggestion and the checklist which is just what I want!
I will try to follow the checklist as much as possible and then PR.  (UPDATED : already open a PR with checklist)
Please let me know if I should close this issue.<|||||>This is great, only the tests are left! No need to close the issue here, we can close this issue once the PR is merged.<|||||>Thanks for your kind words @LysandreJik !
At first, I have no idea how to test. Now I know I have to translate `test_modeling_dpr.py` and see an example on the recent `test_modeling_tf_bart.py` . 
<|||||>@LysandreJik :D
After several hours of testing and debugging, my current model is alreay passed 27 tests :D 
The test run is in Colab here : (in the last cell)
https://colab.research.google.com/drive/1czS_m9zy5k-iSJbzA_DP1k1xAAC_sdkf?usp=sharing

My [current repo](https://github.com/ratthachat/transformers) already contained `test_modeling_tf_dpr.py` 
Could you please suggest me the next step (make a repo update with latest Transformers ?)<|||||>The next steps would be for us to review what you've contributed until now! We'll take a look as soon as possible.<|||||>Thanks Lysandre! I actually have aimed for TFRag . Meanwhile, I will make a new branch and use TFDPR on translating TFRag .<|||||>Close the issue as TFDPR is already merged. Very happy. Thanks a lot everybody!!