Continue pre-training using the example code "run_mlm.py"
Author: pavlion
Pull request: no

## Environment info
- `transformers` version: 4.3.3
- Platform: Linux-5.4.0-65-generic-x86_64-with-glibc2.10  (Ubuntu 18.04)
- Python version: 3.8.8
- PyTorch version (GPU?): 1.7.1 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?:  Yes
- Using distributed or parallel set-up in script?:  No

### Who can help
- tokenizers: @LysandreJik 
- maintained examples (not research project or legacy): @sgugger, @patil-suraj


## Information

Model I am using (Bert, XLNet ...): albert-xlarge-v2

The problem arises when using:
  transformers/examples/language-modeling/run_mlm.py
  (https://github.com/huggingface/transformers/blob/master/examples/language-modeling/run_mlm.py)

The tasks I am working on is:
  continue pre-training on a specific dataset (glue/sst2)

## To reproduce

Steps to reproduce the behavior:
```=
CUDA_VISIBLE_DEVICES=0 python run_mlm.py \
    --model_name_or_path albert-xlarge-v2 \
    --dataset_name "glue" \
    --dataset_config_name "sst2" \
    --do_train \
    --do_eval \
    --output_dir ckpt/pre_training/glue
```

### Error message
```=
Traceback (most recent call last):
  File "src/run_mlm.py", line 447, in <module>
    main()
  File "src/run_mlm.py", line 353, in main
    tokenized_datasets = datasets.map(
  File "/home/robotlab/anaconda3/envs/thesis/lib/python3.8/site-packages/datasets/dataset_dict.py", line 369, in map
    {
  File "/home/robotlab/anaconda3/envs/thesis/lib/python3.8/site-packages/datasets/dataset_dict.py", line 370, in <dictcomp>
    k: dataset.map(
  File "/home/robotlab/anaconda3/envs/thesis/lib/python3.8/site-packages/datasets/arrow_dataset.py", line 1120, in map
    update_data = does_function_return_dict(test_inputs, test_indices)
  File "/home/robotlab/anaconda3/envs/thesis/lib/python3.8/site-packages/datasets/arrow_dataset.py", line 1091, in does_function_return_dict
    function(*fn_args, indices, **fn_kwargs) if with_indices else function(*fn_args, **fn_kwargs)
  File "src/run_mlm.py", line 351, in tokenize_function
    return tokenizer(examples[text_column_name], return_special_tokens_mask=True)
  File "/home/robotlab/anaconda3/envs/thesis/lib/python3.8/site-packages/transformers/tokenization_utils_base.py", line 2286, in __call__
    assert isinstance(text, str) or (
AssertionError: text input must of type `str` (single example), `List[str]` (batch or single pretokenized example) or `List[List[str]]` (batch of pretokenized exampl
es).
```

I skipped the message that relates to the model, as there's no problem with the loading of the model.
Dataset is successfully downloaded. There's a warning above the assertion error
```=
03/02/2021 14:03:44 - WARNING - datasets.builder -   Reusing dataset glue (/home/robotlab/.cache/huggingface/datasets/glue/sst2/1.0.0/7c99657241149a24692c402a5c3f34d
4c9f1df5ac2e4c3759fadea38f6cb29c4)
```


## Expected behavior
When I continue pre-training on other datasets such as 'ag_news', 'dbpedia_14', 'imdb', there's no error and everything is fine.
There are also no "dataset_config_name" in these three datasets. 
However, there's no error when I use   `dataset_name=wikitext` and `dataset_config_name=wikitext-2-raw-v1`    in  `run_mlm.py`

Judging from the error message above, 
it seems like the data format of the SST-2 is wrong so that the datasets can not handled the data correctly.
Any suggestion is highly appreciated!



Comments:
Hi there,

the `run_mlm` script expects an unlabeled text dataset, i.e a dataset with the column `text` in it, if there is no `text` column then it assumes that the first column is the text column.

Here `sst2` is a classification dataset and the first column is `idx`. 
So you could change the script and directly hardcode the `text_column_name`, which is `sentence` for `sst2`.
https://github.com/huggingface/transformers/blob/b013842244df7be96b8cc841491bd1e35e475e36/examples/language-modeling/run_mlm.py#L304

 And also pass the `--line_by_line` argument.

Thanks for your prompt and accurate reply.
This does help!

But I notice that the line_by_line argument is not needed as long as the `text_column_name`  is hard-coded in that line.
Anyways, thanks!