Created by: tpatejko

This PR implements optimization of elementwse_add forward and backward passes. It includes for forward pass:

• MKL VML-based optimization with v?Add then MKL/MKLDNN are used
• Blas-based optimization with VCopy and SAXPY operations when MKL is disabled

For backward pass:

• Blas level 1 VCopy is used for copying dx and dy vectors.

When integral or float16 types, or GPU device are used, the implementation falls back to the default (generic) elementwise_add operation.