`BatchFeature`: Convert `List[np.ndarray]` to `np.ndarray` before converting to pytorch tensors
Author: eladsegal
Pull request: yes

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Fixes #14307


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Comments:
Thanks for your PR!
I think the check needs to be a bit more thorough: we need to also check all the elements of the list are `np.ndarray` (or at least the first one), according to the warning at least.

Thanks @sgugger,
I added a check for the first element, but here are my thoughts about it:
Iterating through the full list doesn't seem helpful - what would you do if half of the list is `List` and the other half is `np.array`? It would still be more efficient to call `np.array` in such a case.
It's the same thing for checking just the first element, but at least the check doesn't have the cost of iterating through the full list.
Therefore, I think that it's best to always pass the list to `np.array` without checking it.

Failure is unrelated and already fixed on master.
Thanks again for your work on this PR!