Training GPT2 does not use GPU
Im using [run_clm.py](https://github.com/huggingface/transformers/blob/master/examples/language-modeling/run_clm.py) to train GPT2. When using the [example from the documetnation](https://github.com/huggingface/transformers/tree/master/examples/language-modeling) it works fine and uses the GPU. But when I start it on my custom dataset it does not use any GPUs. Can you give me a tip on how to get it use the GPUs, or what might be wrong?

this works and uses GPUs:

```
python run_clm.py \
    --model_name_or_path gpt2 \
    --dataset_name wikitext \
    --dataset_config_name wikitext-2-raw-v1 \
    --do_train \
    --do_eval \
    --output_dir /netscratch/nehring/projects/opensubtitles/datadir/tmp \
    --per_device_train_batch_size 2 \
    --per_device_eval_batch_size 2
```


this starts training but it does not use GPUs:

```
python run_clm.py  \
    --model_type gpt2 \
    --tokenizer_name gpt2 \
    --train_file $DATA_PATH/train.txt \
    --validation_file $DATA_PATH/valid.txt \
    --do_train \
    --do_eval \
    --output_dir /netscratch/nehring/projects/opensubtitles/datadir/models/gpt2-small \
    --per_device_train_batch_size 32 \
    --per_device_eval_batch_size 32 \
    --num_train_epochs 10
```

This is my environement as created by `transformers-cli env`. It says that I did not install tensorflow. But when I do `python -c 'import tensorflow as tf; print(tf.__version__)` then the commandline prints "1.15.0". 

```
- `transformers` version: 4.5.0.dev0
- Platform: Linux-5.4.0-65-generic-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.6.9
- PyTorch version (GPU?): 1.8.0 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: this is the problem
- Using distributed or parallel set-up in script?: no
```

This here is part of the output of run_clm.py. it says `_n_gpu=6`. So the GPUs are detected but for some reason they are not used.

```
03/23/2021 18:34:14 - INFO - __main__ -   Training/evaluation parameters TrainingArguments(output_dir=/netscratch/nehring/projects/opensubtitles/datadir/models/gpt2-small, overwrite_output_dir=False, do_train=True, do_eval=True, do_predict=False, evaluation_strategy=IntervalStrategy.NO, prediction_loss_only=False, per_device_train_batch_size=32, per_device_eval_batch_size=32, gradient_accumulation_steps=1, eval_accumulation_steps=None, learning_rate=5e-05, weight_decay=0.0, adam_beta1=0.9, adam_beta2=0.999, adam_epsilon=1e-08, max_grad_norm=1.0, num_train_epochs=10.0, max_steps=-1, lr_scheduler_type=SchedulerType.LINEAR, warmup_ratio=0.0, warmup_steps=0, logging_dir=runs/Mar23_18-34-14_graz, logging_strategy=IntervalStrategy.STEPS, logging_first_step=False, logging_steps=500, save_strategy=IntervalStrategy.STEPS, save_steps=500, save_total_limit=None, no_cuda=False, seed=42, fp16=False, fp16_opt_level=O1, fp16_backend=auto, fp16_full_eval=False, local_rank=-1, tpu_num_cores=None, tpu_metrics_debug=False, debug=False, dataloader_drop_last=False, eval_steps=500, dataloader_num_workers=0, past_index=-1, run_name=/netscratch/nehring/projects/opensubtitles/datadir/models/gpt2-small, disable_tqdm=False, remove_unused_columns=True, label_names=None, load_best_model_at_end=False, metric_for_best_model=None, greater_is_better=None, ignore_data_skip=False, sharded_ddp=[], deepspeed=None, label_smoothing_factor=0.0, adafactor=False, group_by_length=False, report_to=['tensorboard'], ddp_find_unused_parameters=None, dataloader_pin_memory=True, skip_memory_metrics=False, _n_gpu=6)
```
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