Ignore non-instrument tags in loss calculation

[ethman]

Right now we set the GT tags of all non-instrument tags to 0.0, but still compute a loss on the JBX'd audio for those tags, so TagBox creates audio that sets makes the non-instrument tags 0.0 too. But we really should ignore the non-instrument tags all together. So we should set the weight of non-instrument tags to 0.0 when calculating loss for every iteration.