tf.nest package when we dev theese code , flatten and map_structure pack_sequence_as are need eagerly

[mullerhai]

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System information

• Have I written custom code (as opposed to using a stock example script provided in TensorFlow):
• OS Platform and Distribution (e.g., Linux Ubuntu 16.04 x86_64):
• TensorFlow installed from (source or binary):
• TensorFlow version (use command below):
• Java version (i.e., the output of java -version):
• Java command line flags (e.g., GC parameters):
• Python version (if transferring a model trained in Python):
• Bazel version (if compiling from source):
• GCC/Compiler version (if compiling from source):
• CUDA/cuDNN version:
• GPU model and memory:

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Describe the expected behavior

Code to reproduce the issue
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