Update Tensor.hpp and Tensor.cpp

- [#include] Remove duplicated includes, add includes
- [attributes] add default values for mDataType, mImpl and mGrad
- [constructors] Order constructors, change array copy assignment to remove code duplication
- calling grad() does not instanciate the gradient anymore, a dedicated function should do it
- move getAvailableBackends() and operator=(const Tensor&) to cpp file
- Tensor::resize() now handles scalar Tensors