For each row of the target tensor, there is just an active note.
There is a simple example. If batch is size 2 I have two notes.
If note 0 has a shape like [88,5] I have a target like:
[[1,1,1,1,1], # row 0
[0,0,0,0,0] # row 1
.
.
.[0,0,0,0,0]] #row 87

if note 1 has a shape like [88,3] I have a target like:
[[0,0,0], #row 0
[1,1,1], # row 1
.
.
.
.[0,0,0]] #row 87

After concatenate arrays:
[[1,1,1,1,1,0,0,0],
[0,0,0,0,0,1,1,1]
[0,0,0,0,0,0,0,0]
.
.
.
[0,0,0,0,0,0,0]]


This representation is before transpose.