Issue: Text dataset not working with large files

```
Traceback (most recent call last):
  File "examples/language-modeling/run_language_modeling.py", line 333, in <module>
    main()
  File "examples/language-modeling/run_language_modeling.py", line 262, in main
    get_dataset(data_args, tokenizer=tokenizer, cache_dir=model_args.cache_dir) if training_args.do_train else None
  File "examples/language-modeling/run_language_modeling.py", line 144, in get_dataset
    dataset = load_dataset("text", data_files=file_path, split='train+test')
  File "/home/ksjae/.local/lib/python3.7/site-packages/datasets/load.py", line 611, in load_dataset
    ignore_verifications=ignore_verifications,
  File "/home/ksjae/.local/lib/python3.7/site-packages/datasets/builder.py", line 469, in download_and_prepare
    dl_manager=dl_manager, verify_infos=verify_infos, **download_and_prepare_kwargs
  File "/home/ksjae/.local/lib/python3.7/site-packages/datasets/builder.py", line 546, in _download_and_prepare
    self._prepare_split(split_generator, **prepare_split_kwargs)
  File "/home/ksjae/.local/lib/python3.7/site-packages/datasets/builder.py", line 888, in _prepare_split
    for key, table in utils.tqdm(generator, unit=" tables", leave=False, disable=not_verbose):
  File "/home/ksjae/.local/lib/python3.7/site-packages/tqdm/std.py", line 1129, in __iter__
    for obj in iterable:
  File "/home/ksjae/.cache/huggingface/modules/datasets_modules/datasets/text/7e13bc0fa76783d4ef197f079dc8acfe54c3efda980f2c9adfab046ede2f0ff7/text.py", line 104, in _generate_tables
    convert_options=self.config.convert_options,
  File "pyarrow/_csv.pyx", line 714, in pyarrow._csv.read_csv
  File "pyarrow/error.pxi", line 122, in pyarrow.lib.pyarrow_internal_check_status
  File "pyarrow/error.pxi", line 84, in pyarrow.lib.check_status
```

**pyarrow.lib.ArrowInvalid: straddling object straddles two block boundaries (try to increase block size?)**

It gives the same message for both 200MB, 10GB .tx files but not for 700MB file.
Can't upload due to size & copyright problem. sorry.

Comments:
Seems like it works when setting ```block_size=2100000000``` or something arbitrarily large though.

Can you give us some stats on the data files you use as inputs?

Basically ~600MB txt files(UTF-8) * 59. 
contents like ```안녕하세요, 이것은 예제로 한번 말해보는 텍스트입니다. 그냥 이렇다고요.
\n```

Also, it gets stuck for a loooong time at ```Testing the mapped function outputs```, for more than 12 hours(currently ongoing)

It gets stuck while doing `.map()` ? Are you using multiprocessing ?
If you could provide a code snippet it could be very useful

From transformers/examples/language-modeling/run-language-modeling.py :
```
def get_dataset(
    args: DataTrainingArguments,
    tokenizer: PreTrainedTokenizer,
    evaluate: bool = False,
    cache_dir: Optional[str] = None,
):
    file_path = args.eval_data_file if evaluate else args.train_data_file
    if True:
        dataset = load_dataset("text", data_files=glob.glob(file_path), split='train', use_threads=True, 
        ignore_verifications=True, save_infos=True, block_size=104857600)
        dataset = dataset.map(lambda ex: tokenizer(ex["text"], add_special_tokens=True,
                                                truncation=True, max_length=args.block_size), batched=True)
        dataset.set_format(type='torch', columns=['input_ids'])
        return dataset
    if args.line_by_line:
        return LineByLineTextDataset(tokenizer=tokenizer, file_path=file_path, block_size=args.block_size)
    else:
        return TextDataset(
            tokenizer=tokenizer,
            file_path=file_path,
            block_size=args.block_size,
            overwrite_cache=args.overwrite_cache,
            cache_dir=cache_dir,
        )
```

No, I'm not using multiprocessing.

I am not able to reproduce on my side :/

Could you send the version of `datasets` and `pyarrow` you're using ?
Could you try to update the lib and try again ?
Or do you think you could try to reproduce it on google colab ?

Huh, weird. It's fixed on my side too.
But now ```Caching processed dataset``` is taking forever - how can I disable it? Any flags?

Right after `Caching processed dataset`, your function is applied to the dataset and there's a progress bar that shows how much time is left. How much time does it take for you ?

Also caching isn't supposed to slow down your processing. But if you still want to disable it you can do `.map(..., load_from_cache_file=False)`

Ah, it’s much faster now(Takes around 15~20min). 
BTW, any way to set default tensor output as plain tensors with distributed training? The ragged tensors are incompatible with tpustrategy :(

> Ah, it’s much faster now(Takes around 15~20min).

Glad to see that it's faster now. What did you change exactly ?

> BTW, any way to set default tensor output as plain tensors with distributed training? The ragged tensors are incompatible with tpustrategy :(

Oh I didn't know about that. Feel free to open an issue to mention that.
I guess what you can do for now is set the dataset format to numpy instead of tensorflow, and use a wrapper of the dataset that converts the numpy arrays to tf tensors.



>>> Glad to see that it's faster now. What did you change exactly ?
I don't know, it just worked...? Sorry I couldn't be more helpful.

Setting with numpy array is a great idea! Thanks.