Merged commit includes the following changes:

191029891  by xiejw:

    Fix python script file.

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191029336  by isaprykin:

    Add .configure method to the whole hierarchy of DistributionStrategies.

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191026971  by blamb:

    Updates get_started nav and renames the beginner guides to include eager.

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191025863  by mrry:

    Add private Python API for accessing the C++ Session::*Callable API.

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191025795  by mikecase:

    Internal Change.

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191024780  by isaprykin:

    Internal change.

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PiperOrigin-RevId: 191029891