Process 0 very slow when using num_procs with map to tokenizer
Using pre trained HF tokenizer. The result is the same with tokenizer multiprocessing off and on.
I have (absolutely) no idea about the distribution, but since this issue occurs on all of my datasets(regardless of files), I don't think distribution is the problems.

I can use up to 16 cores.
<img width="721" alt="image" src="https://user-images.githubusercontent.com/17930170/97066109-776d0d00-15ed-11eb-8bba-bb4d2e0fcc33.png">
The code I am using is
```

        dataset = load_dataset("text", data_files=[file_path], split='train')
        dataset = dataset.map(lambda ex: tokenizer(ex["text"], add_special_tokens=True,
                                                truncation=True, max_length=args.block_size), num_proc=8)
        dataset.set_format(type='torch', columns=['input_ids'])
        dataset.save_to_disk(file_path+'.arrow')
```

Process 0 very slow when using num_procs with map to tokenizer 
 <img width="721" alt="image" src="https://user-images.githubusercontent.com/17930170/97066109-776d0d00-15ed-11eb-8bba-bb4d2e0fcc33.png">
The code I am using is
```

        dataset = load_dataset("text", data_files=[file_path], split='train')
        dataset = dataset.map(lambda ex: tokenizer(ex["text"], add_special_tokens=True,
                                                truncation=True, max_length=args.block_size), num_proc=8)
        dataset.set_format(type='torch', columns=['input_ids'])
        dataset.save_to_disk(file_path+'.arrow')
```
 
 Using pre trained HF tokenizer. The result is the same with tokenizer multiprocessing off and on.
I have (absolutely) no idea about the distribution, but since this issue occurs on all of my datasets(regardless of files), I don't think distribution is the problems.

I can use up to 16 cores.