clip issue's solution

[junjun3518]

Simpler solution for clipping. Clip noise https://github.com/ming024/FastSpeech2/issues/73#issue-916122995 is caused by multiplying 2^15(half of int16), while int16's range is [-2^15, 2^15-1], it caused during abs max value is positive. This commit solves this by multiplying 2^15-1 instead of 2^15.

[junjun3518]

After this commit & Before this commit