scale_to_z_score_per_key does not work with SparseTensors and key_vocabulary_filename is not None

[cyc]

If you try to run tft.scale_to_z_score_per_key while passing in SparseTensors for x and key, while also setting key_vocabulary_filename to a value other than None, you will get an error message like:

TypeError: Expected string, got <tensorflow.python.framework.sparse_tensor.SparseTensor object at 0x1a7d5e9a0> of type 'SparseTensor' instead.

This exception ultimately originates from here:

https://github.com/tensorflow/transform/blob/v1.3.0/tensorflow_transform/tf_utils.py#L649

The root cause is that key is a SparseTensor instead of a regular Tensor (key.values needs to be passed in instead of key)