AttributeError: 'TrainingArguments' object has no attribute 'resume_from_checkpoint' in training GPT2
## 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: 4.6.0.dev0
- Platform: Windows-10-10.0.19041-SP0
- Python version: 3.8.9
- PyTorch version (GPU?): 1.8.1+cpu (False)
- Tensorflow version (GPU?): 2.4.1 (False)
- Using GPU in script?: yes
- Using distributed or parallel set-up in script?: no

### Who can help
@patrickvonplaten @sgugger 

## Information

Model I am using: GPT-2

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

I am running

```
subprocess.call([python_path,
                     'transformers/examples/pytorch/language-modeling/run_clm.py',
                     '--model_type', 'gpt2',
                     '--model_name_or_path', 'gpt2',
                     '--train_file', 'train.txt',
                     '--do_train',
                     '--validation_file', 'eval.txt',
                     '--do_eval',
                     '--per_gpu_train_batch_size', '1',
                     '--save_steps', '-1',
                     '--num_train_epochs', '2',
                     '--output_dir', output_dir])
```
and after the downloads/initialization phase I obtain the following:

```
Traceback (most recent call last):
  File "C:\Program Files\JetBrains\PyCharm 2021.1.1\plugins\python\helpers\pydev\pydevd.py", line 1483, in _exec
    pydev_imports.execfile(file, globals, locals)  # execute the script
  File "C:\Program Files\JetBrains\PyCharm 2021.1.1\plugins\python\helpers\pydev\_pydev_imps\_pydev_execfile.py", line 18, in execfile
    exec(compile(contents+"\n", file, 'exec'), glob, loc)
  File "transformers/examples/pytorch/language-modeling/run_clm.py", line 459, in <module>
    main()
  File "transformers/examples/pytorch/language-modeling/run_clm.py", line 417, in main
    if training_args.resume_from_checkpoint is not None:
AttributeError: 'TrainingArguments' object has no attribute 'resume_from_checkpoint'
```

I tried to debug it and in effect the `TrainingArguments` object doesn't have the attribute `resume_from_checkpoint`. I see that it is indicated as an optional field, but on google colab, with the same command, it is present, with value None, as expected. 



## Expected behavior

It should start the training, and the weird thing is that running it on Google Colab it works perfectly. 

You don't have the latest master installed and the examples are synced with it. The easiest way is probably to do:
```
pip uninstall transformers
pip install git+https://github.com/huggingface/transformers
```
to upgrade to the latest!

Otherwise you can do an [editable install](https://huggingface.co/transformers/installation.html#editable-install) to avoid those problems in the future.<|||||>You are right, I even tried to pull from the repo, but I didn't realize my installation was not synched with it. Now it should be, using the editable install. 
Thank you!