Change Gather and Slice's attributes into intputs

Context

For Gather and Slice Operator, we didn't keep the same standard as ONNX, some of the inputs were made attributes on Aidge. However these inputs might not be initializers on the onnx model, so loading these attributes while importing the onnx model is not possible. This is why this MR is intending to make both option available for creating a Slice or Gather either by attributes or by inputs.
For Gather operator:

• Indices
  
• GatheredShape
  For Slice:
  
• Starts
  
• Ends
  
• Axes
  

Modified files

• Slice.hpp
• Slice.cpp
• pybind_Slice.cpp
• HorizontalTiling.cpp
• Gather.cpp
• Gather.cpp
• pybind_Gather.cpp

TODO

• DONE : update Gather operator to support initilizing by inputs or attributes.
• DONE : update Slice operator to support initilizing by inputs or attributes.
• DONE : fix HorizontalTiling recipe