Optimizer serialization

[coreylowman]

A common feature to use when building training frameworks is to save the optimizer state along with network state.

There's already a way to convert a D::Vec<E> into a rust Vec<E>, which should make it easy to save Gradients object. However a pain point is that gradients are currently keyed off of UniqueId, which may not be persistent across saving/loading depending on how model is initialized.

Options:

1. Move optimizers away from Gradients and towards using HashMap<String, D::Vec<E>> where String would be the full path to the tensor. This would make it easy to use with TensorCollection, and also require minimal changes to implement this serialization.
2. The API for serializing optimizers could go through a model, so optimizer gets full paths to tensors on save.

[kurnevsky]

It's also good to have a way to clone in memory the state of optimizer. It's useful if you want to pit 2 networks to see if the trained network performs better than the one before training (which is part of alpha go algorithm).

[coreylowman]

@kurnevsky thanks for bringing that up, cloning optimizers is actually really easy with some changes recently. Addressed in the above PR 👍