Fix import error in onnx by adding current directory to sys.path

[ikanamazu]

Problem

The onnx folder was experiencing an import error when trying to import sparseinst,
while the tools folder did not have this issue.
This occurred because the current directory was not added to sys.path,
causing Python to fail when resolving the sparseinst module.

Solution

I modified the implementation in onnx to match the behavior of tools,
ensuring that the current directory is properly included in sys.path.

This fix prevents import errors related to SparseInst and aligns the behavior across both directories.