Fix for github issue #45: num_steps=None is replaced by num_steps=0 by trainer_pb2.py; later, TF raises an error as num_steps must be None or >0.
Fix for github issue #45: num_steps=None is replaced by num_steps=0 by trainer_pb2.py; later, TF raises an error as num_steps must be None or >0.

PiperOrigin-RevId: 245094152

tensorflow/tfx,tensorflow/tfx