Make `RwkvModel` accept `attention_mask` but discard it internally
# What does this PR do?

`RwkvModel` doesn't accept `attention_mask` but the tokenizer (it is `GPTNeoXTokenizer` from the checkpoints) prepares this input. When I try to run the pipeline tests for this model, I get 
```bash
TypeError: forward() got an unexpected keyword argument 'attention_mask'
```

I see it would be quite annoying for people using this model with the default tokenizer. So it might be good to accept it then discard it internally with a warning.
(If `RwkvModel` had its own tokenizer class `RwkvTokenizer`, we could do this inside tokenizer class. But here is not the case)

_The documentation is not available anymore as the PR was closed or merged._