Creates an input_fn that expects a serialized tf.Example fed into a string
placeholder. The function parses the tf.Example according to the provided
feature_spec, and returns all parsed Tensors as features. This input_fn is
for use at serving time, so the labels return value is always None.
a dict of string to VarLenFeature/FixedLenFeature.
the number of query examples expected per batch.
Leave unset for variable batch size (recommended).