Add support for Split.ALL
COLLABORATOR
Honestly, I think we should fix some other issues in Split API before this change. E. g. currently the following will not work, even though it should:
```python
import datasets
datasets.load_dataset("sst", split=datasets.Split.TRAIN+datasets.Split.TEST)  # AssertionError
```

EDIT:
Actually, think it's OK to merge this PR because the fix will not touch this PR's code.
2021-06-26T15:54:38Z
https://github.com/huggingface/datasets/pull/2259#issuecomment-869021755
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2021-06-26T22:33:50Z
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The title says it all.