Add `do_lower_case` handling to GPT2TokenizerFast and descendant tokenizers
I'm currently pretraining a model from scratch based on RoBERTa architecture and am using a custom `ByteLevelBPETokenizer` tokenizer trained on my data with `lowercase` set to `True`. 

For ease of use, I load it with
```
from transformers import RobertaTokenizerFast

tokenizer = RobertaTokenizerFast(
                        'custom-vocab.json',
                        'custom-merges.txt',
                    )
```

The issue, is that unlike `BertTokenizerFast`, `RobertaTokenizerFast` (and it's parent class `GPT2TokenizerFast`) doesn't handle the `do_lower_case` parameter. 

I prefer using a BPE rather than WordPiece on this task so at the moment I make sure to lowercase text before tokenization but it can be error prone in the future.

Looking at the code, it seems `GPT2TokenizerFast` and descendants are the main ones lacking the parameter.

Could it be possible to add this?

I've just openned a pull request that handles this for GPT2TokenizerFast and RobertaTokenizerFast<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
