Add backward implementations

Context

This MR is dedicated to add the backward() implementation for currently supported operators on backend_cuda:

• Add
• AvgPool
• BatchNorm
• FC
• GlobalAvgPool
• MaxPool
• Pad
• Sub
• Reshape

This resolves #8 (moved)

Modified files

• AddImpl.hpp,AddImpl.cpp and Test_AddImpl.cpp add backward Impl for Add;
• AvgPoolingImpl.hpp, AvgPoolingImpl.cpp and Test_AvgPoolingImpl.cpp add backward Impl for AvgPooling;
• BatchNormImpl.hpp, BatchNormImpl.cpp and Test_BatchNormImpl.cpp add backward Impl for BatchNorm;
• FCImpl.hpp, FCImpl.cpp and Test_FCImpl.cpp add backward Impl for FC;
• GlobalAveragePoolingImpl.hpp, GlobalAveragePoolingImpl.cpp and Test_GlobalAveragePoolingImpl.cpp add backward Impl for GlobalAveragePooling;
• MaxPoolingImpl.hpp, MaxPoolingImpl.cpp and Test_MaxPoolingImpl.cpp add backward Impl for MaxPooling;
• PadImpl.hpp, PadImpl.cpp and Test_PadImpl.cpp add backward Impl for Pad;
• SubImpl.hpp and SubImpl.cpp add backward Impl for Sub;
• ReshapeImpl.hpp, ReshapeImpl.cpp and Test_ReshapeImpl.cpp add backward Impl for Reshape;

TODO

• Add backward
• AvgPool backward
• BatchNorm backward
• FC backward
• GlobalAvgPool backward
• MaxPool backward
• Pad backward
• Sub backward
• Reshape backward