[roberta] fix lm_head.decoder.weight ignore_key handling
This PR fixes https://github.com/huggingface/transformers/issues/12426 where `RobertaForMaskedLM` and `RobertaForCausalLM` do:

```
lm_head.decoder.weight = embeddings.word_embeddings.weight
```
and thus `lm_head.decoder.weight` shouldn't be saved or expected to be loaded unless `config.tie_word_embeddings` is `False`.

So this PR:
- adds `_keys_to_ignore_on_save = [r"lm_head.decoder.weight", r"lm_head.decoder.bias"]` - note also added `lm_head.decoder.bias`
- adds `lm_head.decoder.weight` to `_keys_to_ignore_on_load_missing`
- adds a test. For now "private" but may become common down the road if many other models need this feature.
- tweaks `test_save_load_keys_to_ignore_on_save` to be more debug-friendly and not dump the whole `state_dict` - ouch.

Fixes: https://github.com/huggingface/transformers/issues/12426

As discussed in the Issue other models may have the same issue. So let's follow this pattern of making a model "model" and then ask the community to help identify and fix other models.

@LysandreJik 
