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Pin dependency versions for faster pip install, and a few other minor updates.

Bug fixes for GANSynth data loading.

Bugfixes and minor updates.

Splits magenta.music out into a separate package called note_seq (https://github.com/magenta/note-seq).

Replace tf.Dimension.value, which has been deprecated in TF2.

TensorFlow 2 support.

• MusicVAE data pipeline additions and improvements
• TF1 numpy integer workaround in MusicVAE
• A few py3 compatibility fixes

• Onsets and Frames improvements
• Switch to supporting only Python 3

Minor bug fixes.