Lower ReduceLROnPlateau from 0.001 to 1e-6

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Summary

Lowers the min_lr floor of the ReduceLROnPlateau callback in train_model from 0.001 to 1e-6.

Motivation

ReduceLROnPlateau is configured with factor=0.2 and min_lr=0.001. Because the floor (min_lr = 0.001) is close to typical starting learning rates, the scheduler has little or no room to anneal the learning rate.
Lowering the floor to 1e-6 lets the learning rate decay accross several reductions when val_loss plateaus, which help convergence.