NCF Keras: Fail early with TF 1.x + dist strat
NCF Keras: Fail early with TF 1.x + dist strat

This combination does not yet work. Fail early with an explicit message instead of throwing error later on.
alexgorban/models,tombstone/models,tombstone/models,alexgorban/models,tombstone/models,tombstone/models,alexgorban/models,tombstone/models,tombstone/models,alexgorban/models,alexgorban/models