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 those attributes into back into 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

• SliceImpl.hpp
• SliceImpl.cpp
• SliceImpl_forward_kernels.hpp
• Test_SliceImpl.cpp aside from changing the attributes into inputs, to keep the same standards as onnx, the indices in 'Ends' are not included in the sliced part of the tensor.
• GatherImpl.hpp
• GatherImpl.cpp
• GatherImpl_forward_kernels.hpp
• Test_GatherImpl.cpp

TODO

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