

TITLE
Bertabs example: index_select(): Expected dtype int64 for index

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## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 3.3.1
- Platform: Ubuntu 18.04
- Python version: 3.8.0
- PyTorch version (GPU?): torch==1.7.0
- Tensorflow version (GPU?):
- Using GPU in script?:yes
- Using distributed or parallel set-up in script?: no

### Who can help
@patil-suraj

## Information

Following the example in the seq2seq/bertabs readme.md.
I am getting this error:
```
File "/code/tools/transformers/examples/seq2seq/bertabs/modeling_bertabs.py", line 919, in _fast_translate_batch
    alive_seq = torch.cat([alive_seq.index_select(0, select_indices), topk_ids.view(-1, 1)], -1)
RuntimeError: index_select(): Expected dtype int64 for index
```
In a debugger, I see that the 'select_indices' parameter is a tensor of floats.

I don't understand the beam mechanism, so I don't know where to start troubleshooting this.

Any help would be great!

-Tim