run_lm_finetuning.py for GPT2 throw error "Using pad_token, but it is not set yet."
I used the official setting.
```bash
python transformers/examples/run_lm_finetuning.py \
    --output_dir=gpt2_q_model \
    --model_type=gpt2 \
    --model_name_or_path=gpt2 \
    --do_train \
    --train_data_file=txt/{q_files[0]} \
```

But it says the padding id was not set.

```python
ERROR - transformers.tokenization_utils -   Using pad_token, but it is not set yet.
Traceback (most recent call last):
  File "transformers/examples/run_lm_finetuning.py", line 785, in <module>
    main()
  File "transformers/examples/run_lm_finetuning.py", line 735, in main
    global_step, tr_loss = train(args, train_dataset, model, tokenizer)
  File "transformers/examples/run_lm_finetuning.py", line 330, in train
    for step, batch in enumerate(epoch_iterator):
  File "/usr/local/lib/python3.6/dist-packages/tqdm/_tqdm.py", line 979, in __iter__
    for obj in iterable:
  File "/usr/local/lib/python3.6/dist-packages/torch/utils/data/dataloader.py", line 346, in __next__
    data = self._dataset_fetcher.fetch(index)  # may raise StopIteration
  File "/usr/local/lib/python3.6/dist-packages/torch/utils/data/_utils/fetch.py", line 47, in fetch
    return self.collate_fn(data)
  File "transformers/examples/run_lm_finetuning.py", line 231, in collate
    return pad_sequence(examples, batch_first=True, padding_value=tokenizer.pad_token_id)
  File "/usr/local/lib/python3.6/dist-packages/torch/nn/utils/rnn.py", line 384, in pad_sequence
    out_tensor = sequences[0].data.new(*out_dims).fill_(padding_value)
TypeError: fill_() received an invalid combination of arguments - got (NoneType), but expected one of:
 * (Tensor value)
      didn't match because some of the arguments have invalid types: (NoneType)
 * (Number value)
      didn't match because some of the arguments have invalid types: (NoneType)
```
I am having same error as well. Did you manage to fix it or any other updates?<|||||>Can you let me know if 6b4c3ee234db010ae2fb0554c0099fbf1f7f1f51 fixes your issue?<|||||>I encountered this issue and sure enough it is fixed with `6b4c3ee`.

Thanks @julien-c. It's mind blowing that I found the error 15 mins ago, searched here, found that you'd just patched it, and am now able to continue.<|||||>Thanks @julien-c , that fixes it.<|||||>Thanks guys! (and hat/tip @LysandreJik)<|||||>![pad_token_not_Set](https://user-images.githubusercontent.com/6847024/79693641-65e72900-8289-11ea-864b-96d8dd37c6b2.PNG)
<|||||>ValueError: Unable to set proper padding strategy as the tokenizer does not have a padding token. In this case please set the `pad_token` `(tokenizer.pad_token = tokenizer.eos_token e.g.)` or add a new pad token via the function add_special_tokens if you want to use a padding strategy
