[Flax] Correct pt to flax conversion if from base to head
# What does this PR do?

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Currently loading a base model into a head model while using `from_pt=True` is broken in Flax. 
E.g. the following fails:

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

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

FlaxRobertaForMaskedLM.from_pretrained("./", from_pt=True)
```
It's not that trivial to correct it, since the conversion PT => Flax requires some renaming which is a bit "hacky". To solve the problem this PR now always checks whether both the weight name with and withouth base model prefix is expected. If one of the is expected -> then the weight name is correctly changed. 

Tests are added to ensure that all models will be correctly converted from PyTorch in the future.
