Ignore lm_head decoder bias warning
Removes the warning that's currently happening when importing `xlm-roberta-base` with any of the XLM-R models.

Closes https://github.com/huggingface/transformers/issues/9579
Is it normal that this bias is missing?<|||||>By answering your question I realized this could be upstreamed directly in the RoBERTa model, which I just did.

You can take a look at my answer this morning to a similar question: https://github.com/huggingface/transformers/issues/6193#issuecomment-760797867.

XLM-R is an alias of the RoBERTa model, hence why they both need this.