[WIP] Add Serialization for Optimizers and Gradients

[nkoppel]

Adds SerializeWithModel trait to allow serializing Gradients and Optimizers to files. Resolves #785, but technically will not implement SaveToNpz/LoadForNpz for optimizers, because we need an extra model parameter to structure the serialization of Gradients.

TODO

• Implement SerializeWithModel for Adam.
  • Implement TensorCollection for AdamConfig: Did this in a somewhat hacky way by converting the Option<WeightDecay> to an (u64, f64).
• Implement SerializeWithModel for RMSprop.
• Implement SerializeWithModel for Sgd.