Unclear message with `add-new-model-like` and no flax installed
Author: arthurdouillard
Pull request: no

## Environment info

- `transformers` version: 4.18.0.dev0
- Platform: macos high sierra 10.13.4
- Python version: 3.9
- PyTorch version (GPU?): 1.8.1 CPU
- Using GPU in script?: no
- Using distributed or parallel set-up in script?: no

### Who can help

@sgugger @LysandreJik 

## Information

When trying to create a new model with `add-new-model-like` without having flax installed, an error message is raised.

While I think the code should work without flax, especially considering the numerous flags across the codebase (`is_*_available()`) to handle when one of the deep learning framework isn't installed.

The problem arises when using:
* [X] the official example scripts: (give details below)

## To reproduce

Steps to reproduce the behavior:

1. run `transformers-cli add-new-model-like`
2. ask for `deit`
3. see error message

```bash
$ transformers-cli add-new-model-like
What is the model you would like to duplicate? deit
Traceback (most recent call last):
  File "/usr/local/bin/transformers-cli", line 33, in <module>
    sys.exit(load_entry_point('transformers==4.18.0.dev0', 'console_scripts', 'transformers-cli')())
  File "/usr/local/lib/python3.9/site-packages/transformers-4.18.0.dev0-py3.9.egg/transformers/commands/transformers_cli.py", line 52, in main
    service = args.func(args)
  File "/usr/local/lib/python3.9/site-packages/transformers-4.18.0.dev0-py3.9.egg/transformers/commands/add_new_model_like.py", line 1283, in add_new_model_like_command_factory
    return AddNewModelLikeCommand(config_file=args.config_file, path_to_repo=args.path_to_repo)
  File "/usr/local/lib/python3.9/site-packages/transformers-4.18.0.dev0-py3.9.egg/transformers/commands/add_new_model_like.py", line 1314, in __init__
    ) = get_user_input()
  File "/usr/local/lib/python3.9/site-packages/transformers-4.18.0.dev0-py3.9.egg/transformers/commands/add_new_model_like.py", line 1419, in get_user_input
    old_model_info = retrieve_info_for_model(old_model_type)
  File "/usr/local/lib/python3.9/site-packages/transformers-4.18.0.dev0-py3.9.egg/transformers/commands/add_new_model_like.py", line 690, in retrieve_info_for_model
    model_classes = retrieve_model_classes(model_type, frameworks=frameworks)
  File "/usr/local/lib/python3.9/site-packages/transformers-4.18.0.dev0-py3.9.egg/transformers/commands/add_new_model_like.py", line 624, in retrieve_model_classes
    "flax": auto_module.modeling_flax_auto,
  File "/usr/local/lib/python3.9/site-packages/transformers-4.18.0.dev0-py3.9.egg/transformers/file_utils.py", line 2770, in __getattr__
    raise AttributeError(f"module {self.__name__} has no attribute {name}")
AttributeError: module transformers.models.auto has no attribute modeling_flax_auto
```

## Expected behavior

Either work without having to install flax, or raise an error message telling me I need to install flax (which would be a bit annoying if I only intend to contribute to pytorch).

Thank you very much for this library, it's great! ❤️ 


Comments:
Even if you only want to contribute the model for one framework only, you need to have a `dev` install (like for any PR to Transformers). This is all detailed in the [contributing guide](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests)

Noted, thanks.