BERT TF-Lite conversion not working in TensorFlow 2.2.0
# 🐛 Bug
Model conversion succeeds but inputs and outputs are not recognized.

## Information

Model I am using (Bert, XLNet ...): BERT (`bert-base-uncased`)

Language I am using the model on (English, Chinese ...): English

The problem arises when using:
- TensorFlow 2.1.0 + Transformers 2.8.0 - has no problem converting `bert-base-uncased` model to tflite version.
- TensorFlow 2.2.0rc3 + Transformers 2.8.0 - has issues with interoperability.

The tasks I am working on is:
* Convert BERT models to TF-Lite format to use it in mobile apps.
* Trying to use the latest TF-Lite package version for Android in the place of TF-Lite package provided in the repo `huggingface/tflite-android-transformers`.

## To reproduce
Please execute the following code with TensorFlow versions 2.1.0 and 2.2.0-rc3
```
import transformers
from transformers import TFBertModel, BertConfig
import tensorflow as tf
print('TensorFlow version =', tf.__version__)
print('Transformers version =', transformers.__version__)

MODEL_DIR = 'bert-base-uncased'
MAX_SEQ_LEN = 50

# Read the model
config = BertConfig.from_pretrained(MODEL_DIR)
model = TFBertModel(config)


# Set input Spec
input_spec = [
    tf.TensorSpec([1, MAX_SEQ_LEN], tf.int32),
    tf.TensorSpec([1, MAX_SEQ_LEN], tf.int32),
    tf.TensorSpec([1, MAX_SEQ_LEN], tf.int32)
]

model._set_inputs(input_spec, training=False)

print(model.inputs)
print(model.outputs)
```

- For TensorFlow 2.2.0-rc3: Model outputs and inputs are **None**
```
TensorFlow version = 2.2.0-rc3
Transformers version = 2.8.0
None
None
```
- For TensorFlow 2.1.0:
```
TensorFlow version = 2.1.0
Transformers version = 2.8.0
...
[<tf.Tensor 'input_1:0' shape=(None, 50) dtype=int32>, <tf.Tensor 'input_2:0' shape=(None, 50) dtype=int32>, <tf.Tensor 'input_3:0' shape=(None, 50) dtype=int32>]
[<tf.Tensor 'tf_bert_model/Identity:0' shape=(None, 50, 768) dtype=float32>, <tf.Tensor 'tf_bert_model/Identity_1:0' shape=(None, 768) dtype=float32>]
```


## Expected behavior

- I expect the BERT model conversion to work properly to TensorFlow 2.2.0-rc{1/2/3} 
- Preferably BERT should use the default TF-Lite supported layers just like MobileBERT model provided by Google.
- Image - MobileBERT from Google's bert-qa android example (left) vs BERT converted using the above script using TensorFlow v2.1.0 (right)
![MiniLM vs BERT](https://user-images.githubusercontent.com/5736976/80033284-49bdd480-84a1-11ea-805f-b6d26473612e.png)

<!-- A clear and concise description of what you would expect to happen. -->

## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 2.8.0
- Platform: Windows
- Python version: 3.7.7
- PyTorch version (GPU?): 1.4.0 (No GPU)
- Tensorflow version (GPU?): 2.1.0 (working), 2.2.0-rc3 (not working) (no GPU for both versions)
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

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.
