[Add] 'Operator::backend()' member function and move Tensor dependence from header to source file when possible in operator

- Change parent class from Operator to OperatorTensor
- Remove shared and not customed functions from operators
- Uniformize operators behaviour:
    - inputs are set to nullptr at initialization by default
    - parameters whose size can be computed at initialization are (FC, ConvDepthWise)
    - Many more checks in functions with AIDGE_THROW_OR_ABORT()

[Merge] GraphView.hpp
[Doc] update documentation for Node.hpp and GraphView.hpp
[Upd] IOIndex_t from signed<uint16_t> to uint6_t. IONb_t is replaced by IOIndex_t because both types represent the same thing.
[Upd] Make many member-function arguments const for Node and GraphView to improve safety