Centered frames and online preprocessing.

[copybara-service[bot]]

Centered frames and online preprocessing.

• Function for centering and uncentering frames
• Preprocesser to calculate centered F0 and power online.
• pad_end options to compute_power and compute_loudness, preserving default behavior but allowing turning off of end padding.
• Processor to remove center padding from synthesis output.
• vst.gin config for training with centered inputs and outputs.
• Refactor scaling functions in preprocessing.