Found joint.py errors

[donghyeonk]

n_train/20, n_val/20, and n_test/20 cause errors in python3.

I modified
n_train/20 -> n_train//20
n_val/20 -> n_val//20
n_test/20 -> n_test//20
and it works

[zpengc]

in python2, / operator return integer as result, while in python3, / operator return float as result and // operator return integer as result