Update census/tf-keras save_keras_model output

[liufuyang]

Otherwise I have error when running locally:

I0827 10:10:30.505312 4518344128 builder_impl.py:421] SavedModel written to: local-training-output/keras_export/saved_model.pb
Traceback (most recent call last):
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/Users/fuyangl/cloudml-samples/census/tf-keras/trainer/task.py", line 130, in <module>
    train_and_evaluate(args)
  File "/Users/fuyangl/cloudml-samples/census/tf-keras/trainer/task.py", line 123, in train_and_evaluate
    export_path = export_path.decode('utf-8')
AttributeError: 'NoneType' object has no attribute 'decode'

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[liufuyang]

@googlebot I signed it!

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[gogasca]

LGTM, thanks for your contribution