Position ids in RoBERTa
## 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:
- Platform: Ubuntu 20.04
- Python version: Python 3.7.10
- PyTorch version (GPU?): 1.6.0_py3.7_cuda10.1.243_cudnn7.6.3_0
- Tensorflow version (GPU?): No
- Using GPU in script?: Yes
- Using distributed or parallel set-up in script?: No

### Who can help
@LysandreJik 

## Information
Position ids in RoBERTa is not implemented properly.
The problem arises when using:
create_position_ids_from_input_ids in transformers.models.roberta.modeling_roberta.py
Based on this function, position id 0 is never used. This may cause problem when the sequence is long, for example, 512. Token whose id >= 511 will not get its corresponding token ids.


There's a reason why position ids don't start at 0 for RoBERTa, see #5285<|||||>`RoBERTa` never uses 0 and 1 positional ids, in `ROBERTa`, all pad tokens have position id of 1, and the rest of the tokens have position ids in the range `(2, seq_length - num_pad_tokens)`.  It's implemented like this to match the original implementation in fairseq.<|||||>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.

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