Issue: Possible cache miss in datasets

Comment:
I am having a similar issue where only the grouped files are loaded from cache while the tokenized ones aren't. I can confirm both datasets are being stored to file, but only the grouped version is loaded from cache. Not sure what might be going on. But I've tried to remove all kinds of non deterministic behaviour, but still no luck. Thanks for the help!


```python
    # Datasets
    train = sorted(glob(args.data_dir + '*.{}'.format(args.ext)))
    if args.dev_split >= len(train):
        raise ValueError("Not enough dev files")
    dev = []
    state = random.Random(1001)
    for _ in range(args.dev_split):
        dev.append(train.pop(state.randint(0, len(train) - 1)))

    max_seq_length = min(args.max_seq_length, tokenizer.model_max_length)

    def tokenize_function(examples):
        return tokenizer(examples['text'], return_special_tokens_mask=True)

    def group_texts(examples):
        # Concatenate all texts from our dataset and generate chunks of max_seq_length
        concatenated_examples = {k: sum(examples[k], []) for k in examples.keys()}
        total_length = len(concatenated_examples[list(examples.keys())[0]])
        # Truncate (not implementing padding)
        total_length = (total_length // max_seq_length) * max_seq_length
        # Split by chunks of max_seq_length
        result = {
            k: [t[i : i + max_seq_length] for i in range(0, total_length, max_seq_length)]
            for k, t in concatenated_examples.items()
        }
        return result

    datasets = load_dataset(
        'text', name='DBNL', data_files={'train': train[:10], 'dev': dev[:5]}, 
        cache_dir=args.data_cache_dir)
    datasets = datasets.map(tokenize_function, 
        batched=True, remove_columns=['text'], 
        cache_file_names={k: os.path.join(args.data_cache_dir, f'{k}-tokenized') for k in datasets},
        load_from_cache_file=not args.overwrite_cache)
    datasets = datasets.map(group_texts, 
        batched=True,
        cache_file_names={k: os.path.join(args.data_cache_dir, f'{k}-grouped') for k in datasets},
        load_from_cache_file=not args.overwrite_cache)
```

And this is the log

```
04/26/2021 10:26:59 - WARNING - datasets.builder -   Using custom data configuration DBNL-f8d988ad33ccf2c1
04/26/2021 10:26:59 - WARNING - datasets.builder -   Reusing dataset text (/home/manjavacasema/data/.cache/text/DBNL-f8d988ad33ccf2c1/0.0.0/e16f44aa1b321ece1f87b07977cc5d70be93d69b20486d6dacd62e12cf25c9a5)
100%|██████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 13/13 [00:00<00:00, 21.07ba/s]
100%|██████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 40/40 [00:01<00:00, 24.28ba/s]
04/26/2021 10:27:01 - WARNING - datasets.arrow_dataset -   Loading cached processed dataset at /home/manjavacasema/data/.cache/train-grouped
04/26/2021 10:27:01 - WARNING - datasets.arrow_dataset -   Loading cached processed dataset at /home/manjavacasema/data/.cache/dev-grouped
```