Fix warning when collating list of numpy arrays
# What does this PR do?

As reported in #19822 there is a warning issued by PyTorch when we try to batch the result of a feature extractor when `return_tensors="pt"` is not activated. This PR fixes it by stacking the list of NumPy array in a big array first.

Fixes #19822
_The documentation is not available anymore as the PR was closed or merged._