Easy support for searching over all EndModel sub-module hyperparams

[ajratner]

E.g. handle model search not just over EndModel hyperparams, but also over the hyperparams of e.g. an LSTMModule used as input layer.

Right now this can be accomplished using a wrapper function, but would be nice to have a cleaner interface---e.g. where everything model & sub-module class is passed in and then recursively initialized each iteration of the Tuner search