Datasets' cache not re-used
<s>I think this is because the tokenizer is stateful and because the order in which the splits are processed is not deterministic. Because of that, the hash of the tokenizer may change for certain splits, which causes issues with caching.

To fix this we can try making the order of the splits deterministic for map.</s> Actually this is not because of the order of the splits, but most likely because the tokenizer used to process the second split is in a state that has been modified by the first split.

Therefore after reloading the first split from the cache, then the second split can't be reloaded since the tokenizer hasn't seen the first split (and therefore is considered a different tokenizer).

This is a bit trickier to fix, we can explore fixing this next week maybe Sorry didn't have the bandwidth to take care of this yet - will re-assign when I'm diving into it again !
## Describe the bug
For most tokenizers I have tested (e.g. the RoBERTa tokenizer), the data preprocessing cache are not fully reused in the first few runs, although their `.arrow` cache files are in the cache directory.

## Steps to reproduce the bug
Here is a reproducer. The GPT2 tokenizer works perfectly with caching, but not the RoBERTa tokenizer in this example.
```python
from datasets import load_dataset
from transformers import AutoTokenizer

raw_datasets = load_dataset("wikitext", "wikitext-2-raw-v1")
# tokenizer = AutoTokenizer.from_pretrained("gpt2")
tokenizer = AutoTokenizer.from_pretrained("roberta-base")
text_column_name = "text"
column_names = raw_datasets["train"].column_names

def tokenize_function(examples):
    return tokenizer(examples[text_column_name], return_special_tokens_mask=True)

tokenized_datasets = raw_datasets.map(
    tokenize_function,
    batched=True,
    remove_columns=column_names,
    load_from_cache_file=True,
    desc="Running tokenizer on every text in dataset",
)
```

## Expected results
No tokenization would be required after the 1st run. Everything should be loaded from the cache.

## Actual results
Tokenization for some subsets are repeated at the 2nd and 3rd run. Starting from the 4th run, everything are loaded from cache.

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 1.18.3
- Platform: Ubuntu 18.04.6 LTS
- Python version: 3.6.9
- PyArrow version: 6.0.1
