tensorflow bert model can‘t  return all hidden_states 
```
import tensorflow as tf
import tensorflow_datasets
from transformers import *
from tensorflow.keras import layers

configuration = BertConfig.from_pretrained('bert-base-cased', output_hidden_states=True)
# Load dataset, tokenizer, model from pretrained model/vocabulary
tokenizer = BertTokenizer.from_pretrained('bert-base-cased', config=configuration)
# BERT encoder
encoder = TFBertModel.from_pretrained('bert-base-cased', config=configuration)
# Model
input_ids = layers.Input(shape=(100,), dtype=tf.int32)
token_type_ids = layers.Input(shape=(100,), dtype=tf.int32)
attention_mask = layers.Input(shape=(100,), dtype=tf.int32)
outputs = encoder(
    input_ids, token_type_ids=token_type_ids, attention_mask=attention_mask
)
print(outputs)

_, _, hidden_states = outputs[0], outputs[1], outputs[2]
```

output:

```
If your task is similar to the task the model of the ckeckpoint was trained on, you can already use TFBertModel for predic
(<tf.Tensor 'tf_bert_model_2/Identity:0' shape=(None, 100, 768) dtype=float32>, <tf.Tensor 'tf_bert_model_2/Identity_1:0' shape=(None, 768) dtype=float32>)


---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-7-63944e137dcd> in <module>()
     17 )
     18 print(outputs)
---> 19 _, _, hidden_states = outputs[0], outputs[1], outputs[2]

IndexError: tuple index out of range
```

You can check it:
[colab code](https://colab.research.google.com/drive/1zZo2SwBuGoCHsbvL3_fVR5rygWiuR16-?usp=sharing)
I'm encountering the exact same error - and it also happens when trying to output the attention keys. 
I would really like to know where the problem comes from as well! <|||||>@sshleifer <|||||>@julien-c <|||||>transformers==2.7.0 solved the issue in my case<|||||>> transformers==2.7.0 solved the issue in my case

Thanks a lot. It solved my case too<|||||>Thank you so much @VietHoang1710, worked for me - The same problem with tensorflow roberta model, lost a lot of time to this one!<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
