Optimizers should support serialization and deserialization

[caisq]

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TensorFlow.js version

0.6.1

Browser version

Describe the problem or feature request

Optimizers such as tf.train.SGDOptimizer currently do not support serialization (getConfig) and deserialization (fromConfig) they should support that.

Code to reproduce the bug / link to feature request

[DuncanMays]

I work for a company that does distributed computing, and a lot of what we do involves sending tensorflowJS models across the internet. To that end we've developed a library to serialize tfjs models into strings, optimizer and all: https://github.com/DuncanMays/tfjs-serialize.