Flax Save/Load from base model with different name
The following doesn't work correctly at the moment - loading a base model into a head from PyTorch model when the names are different. *E.g.*: 

```python
from transformers import RobertaModel, FlaxRobertaForMaskedLM, RobertaConfig

model = RobertaModel(RobertaConfig())
model.save_pretrained("./")

FlaxRobertaForMaskedLM.from_pretrained("./", from_pt=True)
```

=> Many weights are incorrectly loaded here. I know what the problem is, but it'll require ~1h to solve. Will take a look later today.
cc @patil-suraj <|||||>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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