make dep onnxruntime optional?

[hubutui]

onnxruntime is introduce since #1262 for SAM support.
We need to modify

labelme/labelme/ai/models/segment_anything.py

Line 6 in 67e489b

import onnxruntime

We could use try catch here. If onnxruntime is not available, then SAM is not available, but other function should still work.
Of course, we still need to change other codes, for example, we need to disable "Create AI-Polygon" menu item if onnxruntime is not found. I'm not sure how to do this yet.