pitch correlation is too large

[TeaPoly]

I found FP16 training is hard to converge because pitch correlation result is too large .

PercepNet/src/pitch.cpp

Line 385 in 29d041b

*pitch_corr = xcorr[best_pitch[0]];

But in Jean-Marc Valin's paper A Real-Time Wideband Neural Vocoder at 1.6 kb/s Using LPCNet is between 0 and 1.

LPCNet is designed to operate with 10-ms frames. Each frame includes 18 cepstral coefficients, a pitch period (between 16 and 256 samples), and a pitch correlation (between 0 and 1).

I think it can be improvement.

[TeaPoly]

Here is PR #43. It may need to be verified for correctness.

[nicriverhoo]

@TeaPoly Good idea. It looks like an normalization operation similarly, it is of benefit to quantization definitely, whereas, the dynamic range of pitch-corr is scaled to 0-1, which may not be good for model performance.
From my perspective, the dynamic range of pitch-corr can represent the accuracy of the estimated pitch, in a sense, the model can learn the differences from the (large range)pitch-corr and estimated pitch pair. However, i am not sure about this.