Add MSE missing operators

Context

In order to use MSE with backend cuda, the following operators need to be supported:

• Mul
• ReduceMean
• Pow

Solves #20 (moved)

Modified files

• MulImpl.hpp, MulImpl.cpp and Test_MulImpl.cpp, add Mul forward() and backward();
• ReduceMeanImpl.hpp, ReduceMeanImpl.cpp and Test_ReduceMeanImpl.cpp, add ReduceMean forward() and backward();
• PowImpl.hpp, PowImpl.cpp and Test_PowImpl.cpp, add Pow forward() and backward();

TODO

• Mul forwad
• Mul backward
• ReduceMean forward
• ReduceMean backward
• Pow forward