L2 normalization

https://blog.csdn.net/zqjackking/article/details/69938901（SSD中有L2的实现）

https://github.com/happynear/NormFace

https://github.com/happynear/caffe-windows/blob/ms/src/caffe/proto/caffe.proto

message NormalizeParameter {
	optional string normalize_type = 1 [default = "L2"];
	optional bool fix_gradient = 2 [default = false];
	optional bool bp_norm = 3 [default = false];
	}