Fixes to TF collators
This PR makes a couple of fixes to TF data collators:

1) Fixes an incorrect call to `is_integer` when the default data collator receives `tf.Tensor` input and is outputting `tf.Tensor` as well
2) Prefer "np" tensors rather than "tf" tensors when calling our collators via `to_tf_dataset`. This is because data preprocessing is generally done with `np.ndarray` rather than `tf.Tensor` anyway, and Keras/`tf.data` can do the final conversion to `tf.Tensor` for us.
_The documentation is not available anymore as the PR was closed or merged._