Created by: yaoxuefeng6

Add "out_val_ifempty" attribute in fileter_by_instag op. By setting this attribute, user can determine the output value if all instances in this batch are filtered. Additional, we filter negative label in auc op. With default all zero out value in filter_by_instag op, there might be some invalid data fed to auc op. we can set negative out value in filter_by_instag op to avoid this.