Can't use BatchEncoding in the fit function
I had a [few problems with the `transformers`](https://github.com/huggingface/transformers/issues/5555) and the related [`tensorflow` functionality](https://github.com/tensorflow/tensorflow/issues/41204), but now I've made some progress using trial and error method. 
You can see the issue in this [gist](https://colab.research.google.com/drive/125jJ0qrXGIe6goNrH_Ja7XPZtYp7nMXU?usp=sharing), now I found out that the problem was due to `model.fit(...)` function. 
Here is the first version which causes the error:
```
model.fit(
    x=X_train, #transformers.tokenization_utils_base.BatchEncoding
    y=targets,
    epochs=3
)
```
Output:
```
ValueError: too many values to unpack (expected 2)
```
Second version which at least works:
```
model.fit(
    x=X_train.values(), #dict_values
    y=targets,
    epochs=3
)
```
**So, now the main question is why does it work and is my code correct?** I'm asking because I have seen many code fragments which used `BatchEncoding` as `x` in the `fit`(e.g. [this](https://www.kaggle.com/definedennis/pretrained-bert-with-huggingface-tensorflow-2-1/) and [this](https://github.com/thakursc1/NLPKernels/blob/3bb1fcac60ab8cdc6f2f68a2d9b5b7a477873811/DisasterTweetClassification/Transformer.py)), but I just can't do the same thing.
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