

TITLE
Need suggestion on contributing TFDPR

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# 🌟 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 