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 switch those attributes to inputs so we don't have to load them while creating the nodes.

For Gather operator:

• Indices
  
• GatheredShape
  

For Slice:

• Starts
  
• Ends
  
• Axes
  

Modified files

• slice.py
  
• gather.py
  

TODO

• DONE : replace Slice attributes by inputs
  
• DONE : replace Gather attributes by inputs