Add data augmentation

[vvolhejn]

DDSP and NEWT papers don't mention it, but RAVE does: "We use dequantization, random crop and allpass filters with random coefficients as our data augmentation strategy."

[vvolhejn]

Random crop is actually tricky to do with the way DDSP is set up because the chunks are pre-cut into four-second segments with a fixed overlap, so this would require a larger change where the cutting would happen during data loading. Additionally, we would need to deal with cutting the pitch and loudness signals too.