Fixes tf_default_data_collator sometimes guessing the wrong dtype for labels
Fixes issues reported by @philschmid in [this notebook](https://colab.research.google.com/drive/13nMZPJpgJpqzdl2e5t4cnV26m_l2xwlw?usp=sharing) with `labels` occasionally ending up as `tf.float32` instead of `tf.int64`. 

The underlying cause was that the `dtype` checking code failed in cases when the label for a single example was a Numpy scalar - this caused `isinstance(np.ndarray)` to return `False`. Adding `or isinstance(np.generic)` correctly catches Numpy scalars as well as arrays.
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