Implement tanh activation func

[MikaelVallenet]

I see in neural pkg that tanh is not implemented.
It may be nice to add this activate func that is important today in ML/DL 🚀

[dreaminglwj]

您好，您的邮件已收到，我会尽快查看，^_^

[riverlaker]

邮件已收到，谢谢！                  ——赵淼