Optimize int precision
Optimize int precision to reduce dataset file size.

Close #1973, close #1825, close #861.
@lhoestq, are the tests OK? Some other cases I missed? Do you agree with this approach? I just tested this and it works like a charm :) 

However tokenizing and then setting the format to "torch" to feed the tokens into a model doesn't seem to work anymore, since the pytorch tensors have the int32/int8 precisions instead of int64 that is required as model inputs.

For example:

```python
import torch
from datasets import Dataset
from transformers import BertModel, BertTokenizer

torch.set_grad_enabled(False)

tokenizer = BertTokenizer.from_pretrained("bert-base-uncased")
model = BertModel.from_pretrained("bert-base-uncased")

dataset = Dataset.from_dict({"text": ["hello there !"]})
dataset = dataset.map(tokenizer, input_columns="text", remove_columns=dataset.column_names)
dataset = dataset.with_format("torch")

print(dataset.features)
# {'attention_mask': Sequence(feature=Value(dtype='int8', id=None), length=-1, id=None),
#  'input_ids': Sequence(feature=Value(dtype='int64', id=None), length=-1, id=None),  # this should be int32 though
#  'token_type_ids': Sequence(feature=Value(dtype='int8', id=None), length=-1, id=None)}

model(**dataset[:1])
# RuntimeError: Expected tensor for argument #1 'indices' to have scalar type Long; but got torch.CharTensor instead (while checking arguments for embedding)

dataset = dataset.with_format("torch", dtype=torch.int64)

model(**dataset[:1])
# works as expected
```

Pinging @sgugger here to make sure we take the right decision here.

Do we want the "torch" format to always return int64 ? Or does it have to keep the precision defined by the `dataset.features` 
 and therefore we would need to specify "torch" with `dtype=torch.int64` ? From a user perspective, I think it's fine if the "torch" format converts all ints types to `torch.int64` by default since it's what the model will need almost all the time. I don't see a case where you would want to keep the low precision at the top of my head, and one can always write a custom transform for an edge case. Sounds good to me !
For consistency maybe we should make the float precision fixed as well (float32, I guess) Yes, that would be the one used by default. Do we have the same requirements for TensorFlow? Yes I we should do the same for tensorflow as well since tf models would have the same issue

Thanks for adding this :) @lhoestq I think this PR is ready... :)