AttributeError: 'GPT2LMHeadModel' object has no attribute 'generate'
## 🐛 Bug

<!-- Important information -->

The example script `run_generation.py` is broken with the error message `AttributeError: 'GPT2LMHeadModel' object has no attribute 'generate'`


## To Reproduce

Steps to reproduce the behavior:

1. In a terminal, cd to `transformers/examples` and then `python run_generation.py --model_type=gpt2 --model_name_or_path=gpt2`
2. After the model binary is downloaded to cache, enter anything when prompted "`Model prompt >>>`"
3. And then you will see the error:

```
Traceback (most recent call last):
  File "run_generation.py", line 236, in <module>
    main()
  File "run_generation.py", line 216, in main
    output_sequences = model.generate(
  File "C:\Anaconda3\lib\site-packages\torch\nn\modules\module.py", line 585, in __getattr__
    type(self).__name__, name))
AttributeError: 'GPT2LMHeadModel' object has no attribute 'generate'
```

<!-- If you have a code sample, error messages, stack traces, please provide it here as well. -->

## Expected behavior

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

## Environment

* OS: Windows 10
* Python version: 3.7.3
* PyTorch version: 1.3.1
* PyTorch Transformers version (or branch): 2.2.2
* Using GPU ? N/A
* Distributed of parallel setup ? N/A
* Any other relevant information:

I'm running the latest version of `run_generation.py`. Here is the permanent link: https://github.com/huggingface/transformers/blob/ce50305e5b8c8748b81b0c8f5539a337b6a995b9/examples/run_generation.py

## Additional context

<!-- Add any other context about the problem here. -->

Same problem<|||||>I have found the reason.

So it turns out that the `generate()` method of the `PreTrainedModel` class is newly added, even newer than the latest release (2.3.0). Quite understandable since this library is iterating very fast.

So to make `run_generation.py` work, you can install this library like this:

- Clone the repo to your computer
- cd into the repo
- Run `pip install -e .` (don't forget the dot)
- Re-run `run_generation.py`

I'll leave this ticket open until the `generate()` method is incorporated into the latest release.<|||||>@jsh9's solution worked for me!

Also, if you want to avoid doing the manual steps, you can just `pip install` directly from the `master` branch by running:

```bash
pip install git+https://github.com/huggingface/transformers.git@master#egg=transformers
```
<|||||>i was getting the same error then i used repository before 7days which is working fine for me 
`!wget https://github.com/huggingface/transformers/archive/f09d9996413f2b265f1c672d7a4b438e4c5099c4.zip`

then unzip with

`!unzip file_name.zip`

there is some bugs in recent update, hope they fix it soon<|||||>@Weenkus's way worked for me. In `requirements.txt` you can use;
```
-e git+https://github.com/huggingface/transformers.git@master#egg=transformers
```

(all on one line)<|||||>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.
