Revert shared cache. Doesn't save enough memory to be worth locking the system for it.

This reverts commit 52861f20f427ffa403966583ab2b637fd264228c.
new parameter init_model to initialize the network parameters from an existing model.
(use cases: continue training with more epochs, or save memory by using different subsets of data for each epoch)
support for networks with single hidden layer

(set num_hidden to 0; num_output_embeddings will be size of remaining layer)