Fix numpy stacking
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This PR also fixed a bug where numpy arrays were returned instead of pytorch tensors when getting with a clumn as a key.
2020-08-11T14:56:50Z
https://github.com/huggingface/datasets/pull/494#issuecomment-671996932
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2020-08-11T14:56:50Z
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When getting items using a column name as a key, numpy arrays were not stacked.
I fixed that and added some tests.

There is another issue that still needs to be fixed though: when getting items using a column name as a key, pytorch tensors are not stacked (it outputs a list of tensors). This PR should help with the to fix this issue.