avoid useless computations if coverage_norm or length_norm = 0
avoid useless computations if coverage_norm or length_norm = 0

jungikim/OpenNMT,OpenNMT/OpenNMT,da03/OpenNMT,monsieurzhang/OpenNMT,jsenellart/OpenNMT,jungikim/OpenNMT,da03/OpenNMT,da03/OpenNMT,jsenellart-systran/OpenNMT,OpenNMT/OpenNMT,monsieurzhang/OpenNMT,jsenellart/OpenNMT,jungikim/OpenNMT,monsieurzhang/OpenNMT,jsenellart-systran/OpenNMT,jsenellart/OpenNMT,jsenellart-systran/OpenNMT,OpenNMT/OpenNMT