run_clm.py does not work with any other block_size other than 1024
**Note:** This issue can be fixed with one character change as described in last section.

## Environment info
- `transformers` version: 4.4.0.dev0
- Platform: Windows-10-10.0.19041-SP0
- Python version: 3.8.5
- PyTorch version (GPU?): 1.7.1 (True)
- Tensorflow version (GPU?): 2.4.1 (False)
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No


### Who can help
- maintained examples (not research project or legacy): @sgugger, @patil-suraj

## Information

Model I am using (Bert, XLNet ...): GPT2

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:
* [X] an official GLUE/SQUaD task: Causal language modelling
* [ ] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

1. Use GPT based model which has block_size different than 1024
2. Try to train it or fine tune with run_clm.py setting setting block_size in data_args.

In GPU mode, you will get following error:
```
RuntimeError: CUDA error: CUBLAS_STATUS_NOT_INITIALIZED when calling `cublasCreate(handle)`
```

In CPU mode, you will get following error:

```
index out of range in self
```

## Expected behavior
Above error should not occur.

## Cause and Proposed Fix

The issue is because block_size on [line 337](https://github.com/huggingface/transformers/blob/master/examples/language-modeling/run_clm.py#L337) always get set to 1024 because of wrong indentation:

```
    if data_args.block_size is None:
        block_size = tokenizer.model_max_length
        if block_size > 1024:
            logger.warn(
                f"The tokenizer picked seems to have a very large `model_max_length` ({tokenizer.model_max_length}). "
                "Picking 1024 instead. You can change that default value by passing --block_size xxx."
            )
        block_size = 1024 # <<< THIS LINE NEEDS TO BE INTENDED!!!
```
 So just tabbing that line should fix the issue.
Indeed! Would you mind making a PR with that change since you found the correct fix?<|||||>Sure, I'll get that prepared!<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.