Different objects are returned from calls that should be returning the same kind of object.
`load_dataset` returns a `DatasetDict` object unless `split`  is defined, in which case it returns a `Dataset` (or a list of datasets if `split` is a list). We've discussed dropping `DatasetDict` from the API in https://github.com/huggingface/datasets/issues/5189 to always return the same type in `load_dataset` and support datasets without (explicit) splits. IIRC the main discussion point is deciding what to return when loading a dataset with multiple splits, but `split` is not specified. What would you expect as a return value in that scenario? > `load_dataset` returns a `DatasetDict` object unless `split` is defined, in which case it returns a `Dataset` (or a list of datasets if `split` is a list). We've discussed dropping `DatasetDict` from the API in #5189 to always return the same type in `load_dataset` and support datasets without (explicit) splits. IIRC the main discussion point is deciding what to return when loading a dataset with multiple splits, but `split` is not specified. What would you expect as a return value in that scenario?

Wouldn't a dataset with multiple splits already have keys and their related data arrays?

Lets say the dataset has "train" : trainset, "valid": validset and "test": testset

So a dictionary can be returned,, i.e.

{ 
"train": trainset,
"valid": validset,
"test": testset
}

if a split is provided split=['train[:80%]', 'valid[80%:90%]', 'test[90%:100%]']

would also return the same dictionary as above.

split='train[:10%]' should return the same value as split=['train[:10%]']

{
"train": trainset
}
 
### Describe the bug

        1.  dataset = load_dataset("togethercomputer/RedPajama-Data-1T-Sample", cache_dir=training_args.cache_dir, split='train[:1%]')
        2.  dataset = load_dataset("togethercomputer/RedPajama-Data-1T-Sample", cache_dir=training_args.cache_dir)

The only difference I would expect these calls to have is the size of the dataset.

But, while 2. returns a dictionary with "train" key in it, 1. returns a dataset WITHOUT any initial "train" keyword.

Both calls are to be used within exactly the same context. They should return identically structured datasets of different size.

### Steps to reproduce the bug

See above.

### Expected behavior

Expect both calls to return the same structured Dataset structure but with different number of elements, i.e. call 1. should have 1% of the data of the call 2.0

### Environment info

Ubuntu 20.04
gcc 9.x.x.

It is really irrelevant.