Adding GPU automatic mixed precision training

[vinhngx]

Automatic Mixed Precision training on GPU for TensorFlow has been recently introduced:

https://medium.com/tensorflow/automatic-mixed-precision-in-tensorflow-for-faster-ai-training-on-nvidia-gpus-6033234b2540

Automatic mixed precision training makes use of both FP32 and FP16 precisions where appropriate. FP16 operations can leverage the Tensor cores on NVIDIA GPUs (Volta, Turing or newer architectures) for improved throughput. Mixed precision training also often allows larger batch sizes.

This PR adds GPU automatic mixed precision training to tensorflow-wavenet via passing the flags value --auto_mixed_precision=True.

python train.py --data_dir=/path/to/data/ --auto_mixed_precision=True

To learn more about mixed precision and how it works:

Overview of Automatic Mixed Precision for Deep Learning
NVIDIA Mixed Precision Training Documentation
NVIDIA Deep Learning Performance Guide