Language model: replace the optimizer and LR-decay scheduler

Replace the original "homebrew" optimizer and LR-decay schedule with
PyTorch's SGD and ReduceLROnPlateau.
SGD with momentum=0 and weight_decay=0, and ReduceLROnPlateau with
patience=0 and factor=0.5 will give the same behavior as in the
original PyTorch example.

Having a standard optimizer and LR-decay schedule gives us the
flexibility to experiment with these during the training process.