[Flax] Non working model when exporting to Huggingface
I have trained a RoBERTa base Norwegian according to instructions given at https://github.com/huggingface/transformers/tree/master/examples/flax/language-modeling#masked-language-modeling.

The final mlm accuracy is 0.63, indicating a working model.

I am trying to load the model, and export til to PyTorch (or TF) for using the inference widget on Hugging Face.

The following code runs without errors:

from transformers import AutoTokenizer, RobertaForMaskedLM
model = RobertaForMaskedLM.from_pretrained('model_dir', from_flax=True)
tokenizer = AutoTokenizer.from_pretrained('model_dir')
model.save_frompretrained('.')
tokenizer.save_frompretrained('.')
Example widget here: https://huggingface.co/pere/norwegian-roberta-base?text=Dette+er+en+%3Cmask%3E.

The outputs makes absolutely no sense. What is the correct way of exporting a Flax model (with and without the MLM head)?
cc @patrickvonplaten @patil-suraj <|||||>Hi @peregilk, how did you save the flax Roberta model using the `FlaxRobertaModel` class or using the `FlaxRobertaForMaskedLM` class? 
I tried loading the flax model with
```python
fx_model = FlaxRobertaForMaskedLM.from_pretrained("pere/norwegian-roberta-base")
```
and it gives this warning 
```
Some weights of FlaxRobertaForMaskedLM were not initialized from the model checkpoint at pere/norwegian-roberta-base and are newly initialized: {('lm_head', 'dense', 'kernel'), ('lm_head', 'layer_norm', 'bias'), ('lm_head', 'layer_norm', 'scale'), ('lm_head', 'bias'), ('lm_head', 'dense', 'bias')}
You should probably TRAIN this model on a down-stream task to be able to use it for predictions and inference.
```

which means the model was not saved with `FlaxRobertaForMaskedLM`, so the `lm_head` is missing. In which case it is randomly initialized, which might explain the above behavior.

Please make sure you use the `*ForMaskedLM` class to save, load MLM models. Thanks!<|||||>Thanks for the feedback @patil-suraj. I think you are absolutely right that the uploaded model does not contain the LM head. The issue is then probably related to the loading.

I have tried both these alternatives:
```
model = RobertaForMaskedLM.from_pretrained('model_dir', from_flax=True)
model = FlaxRobertaForMaskedLM.from_pretrained('model_dir')
```

Both giving roughly the same "random" result. I actually notice errors from both these methods when loading. Slightly different, but both pointing at the lm head not being loaded correctly:

```
- This IS NOT expected if you are initializing RobertaForMaskedLM from a Flax model that you expect to be exactly identical (e.g. initializing a BertForSequenceClassification model from a FlaxBertForSequenceClassification model).
This is the version that is currently pushed, and you are probably right the head is not included.
```


```
Some weights of FlaxRobertaForMaskedLM were not initialized from the model checkpoint at . and are newly initialized: {('lm_head', 'layer_norm', 'scale'), ('lm_head', 'bias'), ('lm_head', 'dense', 'kernel'), ('lm_head', 'layer_norm', 'bias'), ('lm_head', 'dense', 'bias')}
```

How can I load the Flax model with the LM head?
<|||||>Related: How do I check that `flax_model.msgpack` actually is saved with the LM head?<|||||>@peregilk - I seems like the uploaded flax model weights are not correct. When running:

```python
from transformers import FlaxRobertaForMaskedLM
model = FlaxRobertaForMaskedLM.from_pretrained("pere/norwegian-roberta-base")
```

The lm weights are missing. <|||||>I think this commit: https://huggingface.co/pere/norwegian-roberta-base/commit/163f3993e02f514ce29901020a7ec73958fb415b was where they were deleted. Can you revert this change and instead use the Flax model weights from this commit: https://huggingface.co/pere/norwegian-roberta-base/commit/506fb71065742c28f1efe8ccb9cae48ddcc562ad .

If you convert the flax weights of this commit: https://huggingface.co/pere/norwegian-roberta-base/commit/506fb71065742c28f1efe8ccb9cae48ddcc562ad to a `RobertaForMaskedLM` pytorch model, the widget should work correctly<|||||>@patrickvonplaten. Awesome! Works perfectly! I must have overwritten this in my first attempts. Did not realise that loading and saving FlaxRobertaModel removes the LM head. But it makes perfectly sense that it does.<|||||>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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