Ddsp

Add custom cumsum function that doesn't accumulate phase errors like tf.cumsum.

* Updated pitch detection metrics (RPA, RCA)
* Sinusoidal Synthesizer
* Warm starting models (model_dir -> save_dir, restore_dir)

Small fixes to bugs introduced by refactor :).

Some bug fixes and a refactor of train_util and eval_util.

* New data normalization in the demo colab notebooks.
* Tiny model config.
* Most (but not all) of the variable sample rate PRs.
* Tests and bug fixes.

Simplify and refactor RnnFcDecoder.

* Requires old models to add a single line to their operative gin configs, or --gin_param, `RnnFcDecoder.input_keys = ('f0_scaled', 'ld_scaled')`