Issue in generating samples for text generation
# 🐛 Bug

## Information

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

```python
def generate_samples(args, model, prompt_text):
  """Generating sampling for the provided prompt using the provided model."""
  set_seed(args.seed)

  _, _, tokenizer_class = run_language_modeling.MODEL_CLASSES[args.model_type]
  tokenizer = tokenizer_class.from_pretrained(args.model_name_or_path, cache_dir=None)

  requires_preprocessing = args.model_type in run_generation.PREPROCESSING_FUNCTIONS.keys()
  encoded_prompt = tokenizer.encode(prompt_text, add_special_tokens=False, return_tensors="pt")
  encoded_prompt = encoded_prompt.to(args.device)
```

Error: run_language_modeling has no 'MODEL_CLASSES'

Language I am using the model on (English):



The file `run_language_modeling.py` does indeed not have a variable called `MODEL_CLASSES`. Can you explain what you are trying to do exactly?