As title.
The source code for "Enabling factorized piano music modeling and generation with the MAESTRO dataset" is listed as unknown.
It is now a part of Magenta and is found here.
There are also 2 relevant papers in the same repo but not listed here:
Onsets and Frames: Dual-Objective Piano Transcription, and
Improving Perceptual Quality of Drum Transcription with the Expanded Groove MIDI Dataset.
It would be great to add this repo and these 2 papers to the list.