Extend Conf Schema for Network Optimizers

similarly to optimizer:sgd {...} we want to allow the definition of network_optimizer parameters. Since depending on the concrete optimizer it might have varying parameters, e.g. alpha, beta in the case of efficient net, it should be written in a nested way similarly to the optimizer example as network_optimizer:efficientnet {efficientnet specific params come here} or network_optimizer:adanet{adanetspecific params come here}