Use dense Adam by convert indexedslices to tensor in DIEN

[retonym]

The new implementation converts indexedslices to tensor in Adam optimizer. Tensorflow use a single op "apply_adam" to deal with tensors, while use many small ops to deal wth indexedslices in sparse Adam implementation. To use tensors avoids to load these small ops repeatedly and saves time. In my experiment, this change improves about 12% throughput with batch size 128 on Intel Xeon 8260 processors.

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[Zantares]

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[retonym]

I have move the changes of this PR to DIEN_TF2 folder. It will not influence the code in the original DIEN folder.