Update vit operators

The updates in this MR concern the operators Gather and Slice.

• Gather: changing the second input (indexes) into attributes (indexes and gatheredShape)
  - indexes: vector containing the indexes of the element we want to gather along the Axis
  - gatheredShape: vector containing the shape of the gathered elements. This is necessary because in ONNX, the indexes can be a singular scalar and if it is the case then the dimension on which we do the gather operation will be removed. However if the indexes are an array then the dimension of Axis would equal the number of gathered elements.
  
• Slice: changed types from std::int32_t to std::int64_t since that is the type mainly used in ONNX.