Matmul rework

The behavior of the reworked MatMul on the arguments in the following way.
- If both arguments are 2-D they are multiplied like conventional matrices.
- If either argument is N-D, N > 2, it is treated as a stack of matrices residing in the last two indexes.
Exp: For the MatMul of tensor A of shape (3,3,2) with tensor B of shape (3,2,4), A is a stack of three 2D matrices each of shape (3,2), and B is a stack of 3 2D matrices, each of shape (2,4).

include/aidge/backend/cpu/data/GetCPUPtr.h

@@ -15,9 +15,9 @@
 #include "aidge/data/Tensor.hpp"
 
 namespace Aidge {
-inline void *getCPUPtr(std::shared_ptr<Aidge::Data> const &data) {
+inline void *getCPUPtr(std::shared_ptr<Aidge::Data> const &data, const std::size_t offset = 0) {
   const auto tensor = std::static_pointer_cast<Tensor>(data);
-  return tensor->getImpl()->hostPtr(tensor->getImplOffset());
+  return tensor->getImpl()->hostPtr(tensor->getImplOffset() + offset);
 }
 } // namespace Aidge