Job to create new "virtual" stem: `.instrumental`.

[CharlesHolbrow]

Make sure to handle dirs like this&authuser=0&project=klay-training&prefix=&forceOnObjectsSortingFiltering=false)

Run on:

• gs://klay-datasets-001/glucose-karaoke-003
• gs://klay-datasets-001/mtg-jamendo-90s-crop

Some remarks:

• The glucose-karaoke dataset has enumerated stems, for example the other stem can have multiple sub stems which are annotated as other, other-1, other-2, ...
• The vocals stem should always be ignored, this includes the vocals-1, vocals-2 stems

[CharlesHolbrow]

closing in favor of https://github.com/klay-music/klay-beam-jobs/issues/2