Performance issue in the definition of transform_targets_for_output, yolov3_tf2/dataset.py(P1)

[DLPerf]

Hello, I found a performance issue in the definition of transform_targets_for_output, yolov3_tf2/dataset.py, tf.range(tf.shape(y_true)[1]) will be calculated repeatedly during program execution, resulting in reduced efficiency. I think it should be created before the loop.

Looking forward to your reply. Btw, I am very glad to create a PR to fix it if you are too busy.