Fix ONNX init

[awsr]

ORTPipelinePart was only used from optimum versions 1.23.0 through 1.25.3

Description

• Fixes ONNX initialization module 'optimum.onnxruntime.modeling_diffusion' has no attribute 'ORTPipelinePart'
• Added a version check so that the import is only performed when using a version that utilized it.
• Fix initially made for https://github.com/lshqqytiger/stable-diffusion-webui-amdgpu-forge/issues/106 but it also affects this version.

Checklist:

• [x] I have read contributing wiki page (note, there is no dev branch)
• [x] I have performed a self-review of my own code
• [x] My code follows the style guidelines
• [x] My code passes tests