[onnx] Fix ONNX import of some operators

The ONNX imports of some operators are broken this includes:

• ReduceMean
• ReduceSum
• Slice
• Split
• Squeeze
• Shape
• Unsqueeze
• Reshape
• LeakyReLU
• Transpose

TODO

!84 (merged)

• ReduceMean
• ReduceSum
• Squeeze
• Unsqueeze
• Transpose

!76 (merged)

• Slice
• Split
• Shape
• LeakyReLU
• Reshape