OK, I see.
The issue is setting your labels to one.
label[0, 20:32, 80:150, 80:150] = 1

Since you are missing a dimension (label was initialized as torch.zeros(1, 1, 50, 300, 300)), you are trying to index 20:32 in dim1, which will return an empty slice.
Thus labels only contain zeros without the manually assigned target in the volume.
Change the line to:
label[0, 0, 20:32, 80:150, 80:150] = 1

and run it again.