[bugfix] Fix for importing the estimator_export decorator from tensorflow_estimator

[othakkar]

Due to a recent commit in TensorFlow (tensorflow/tensorflow@3837d0f) that removes support for the estimator_export decorator from tf_export inside TensorFlow, several models such as MMoE fail to run with the following error:
ImportError: cannot import name 'estimator_export' from 'tensorflow.python.util.tf_export'.

As a solution, 'estimator_export' now needs to be imported from within the tensorflow_estimator package itself.

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✅ othakkar
❌ chengmengli06
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