Add MSE support on backend_cuda

In order to run learning tutorial with backend_cuda, we need to support the operators used in the MSE:

• ReduceMean
• Pow
• Mul

assigned to @hrouis

added Feature 🚀 label

changed the description

added StatusWork in Progress label

mentioned in merge request !31 (closed)

added StatusReview Ready label and removed StatusWork in Progress label

mentioned in merge request !33 (merged)

closed

removed StatusReview Ready label