Add conversion from TF to PT for Tapas retrieval models
# What does this PR do?

Table Retrieval models based on Tapas as described [here](https://arxiv.org/pdf/2103.12011.pdf) just got published in the [Tapas repository](https://github.com/google-research/tapas). The existing conversion function does not work with the retrieval models, so I added support to convert them to Pytorch.
Unfortunately, this only converts the language model without the down projection layer. However, I think this might still be useful to some people who, for instance, want to fine-tune the pre-trained models. 
Unfortunately, I do not have the time at the moment to add the down projection layer myself.


## Who can review?

@NielsRogge 

Thanks for this, it's certainly something I'd like to add in the future. The TAPAS team seems quite active, they released [yet another paper involving TAPAS](https://arxiv.org/abs/2106.00479) (to work on larger tables).<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>@NielsRogge I just quickly reviewed this script and it looks fine. Is it possible to know why this PR became stall and was automatically closed? Is there anything wrong with the script I should be aware of? 

I'm currently working on finetuning the TAPAS retrieval model for a research project, just wanted to have your thoughts on this before running the script and uploading the model to the Huggingface hub.<|||||>@jonathanherzig Just wanted to confirm with you, in this case `bert` is the table encoder and `bert_1` is the question encoder, right? <|||||>Hi @xhlulu ,
Sorry, but I not familiar with the implementation details in this version of TAPAS... probably @NielsRogge  can help.

Best,
Jonathan<|||||>No worries, thanks Jonathan!