MMBT doesn't inherit from nn.Module
# 🐛 Bug

## Information

Model I am using (Bert, XLNet ...): MMBT

Language I am using the model on (English, Chinese ...): not related

The problem arises when using:
* [ ] the official example scripts: (give details below)
* [x] my own modified scripts: (give details below)

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [x] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

Minimal reproduction:
```python
from transformers import MMBTConfig, MMBTModel, AutoConfig, AutoModel

electra_config = AutoConfig.from_pretrained("google/electra-small-discriminator")
mmbt_config = MMBTConfig(electra_config)

model = AutoModel.from_config(electra_config)

mmbt = MMBTModel(mmbt_config, model, None)

mmbt()
```

output:
```
Traceback (most recent call last):
  File "mmbt_debug.py", line 11, in <module>
    mmbt()
TypeError: 'MMBTModel' object is not callable
```

You can see in the [source code](https://github.com/huggingface/transformers/blob/master/src/transformers/modeling_mmbt.py#L152) that it's currently only inheriting from `ModuleUtilsMixin`, but not `torch.nn.Module`

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## Expected behavior

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

We should be seeing a downstream error since I didn't pass in a real modal encoder or any input.  It should at least call `forward()`

## 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: 2.9.1 (also tried 2.10.0)
- Platform: Darwin-19.4.0-x86_64-i386-64bit
- Python version: 3.6.10
- PyTorch version (GPU?): 1.5.0 (False)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: no (doesn't matter)
- Using distributed or parallel set-up in script?: no (doesn't matter)
