Created by: parap1uie-s

sigmoid_focal_loss的gamma参数默认为2，int类型 若外部调用时使用默认gamma值，则会出现错误：

paddle.fluid.core_avx.EnforceNotMet: Cannot get attribute gamma by type float, its type is int at [/paddle/paddle/fluid/framework/attribute.h:42]

因此将gamma默认值修改为"2."