TypeError: add_() takes 1 positional argument but 2 were given
# 🐛 Bug

## Information

I was trying to reproduce the GLUE fine-tuning example (https://huggingface.co/transformers/examples.html#fine-tuning-example) when I ran into this error:

```
  File "~/anaconda3/lib/python3.7/site-packages/transformers/optimization.py", line 155, in step
    exp_avg.mul_(beta1).add_(grad, 1.0 - beta1)
TypeError: add_() takes 1 positional argument but 2 were given
```

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

The tasks I am working on is:
* [x] an official GLUE/SQUaD task: (give the name)


## To reproduce

Steps to reproduce the behavior:
Follow the official guide example here:  https://huggingface.co/transformers/examples.html#fine-tuning-example

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## Expected behavior

<!-- 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: 2.9.1
- Platform: Linux 5.4.0-29-generic x86_64 Ubuntu 20.04 LTS
- Python version: 3.7.6
- PyTorch version (GPU?): 1.5.0 (Yes)
- Tensorflow version (GPU?): 
- Using GPU in script?: Yes, 1
- Using distributed or parallel set-up in script?: 

This was fixed on master on Friday, can you try pulling from master again?