Update optimize_pipeline.py to use __name__ detection (#15866)

### Description
<!-- Describe your changes. -->

Use `__name__` detection in `optimize_pipeline.py`.

### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->

It prevents unwanted execution of `main` when importing the file.

onnxruntime/python/tools/transformers/models/stable_diffusion/optimize_pipeline.py

@@ -312,4 +312,5 @@ def main():
     )
 
 
-main()
+if __name__ == "__main__":
+    main()