Updates `ModelScopeImageGenerator` in `nodes.py` to:
- Use `v1/images/generations` with `X-ModelScope-Async-Mode: true`.
- Implement polling logic for task completion.
- Update input parameters to match API documentation (steps, guidance, seed, loras, size).
- Support `loras` as string or JSON dict.
- Renamed `PIL_Image` local variable to `pil_image` to match naming conventions.
- Introduced constants `PLACEHOLDER_MODEL_ID`, `PLACEHOLDER_API_KEY`, and `CONTENT_TYPE_JSON` to remove duplicated literals.
- Removed `verify=False` from `requests.post` calls to enable SSL certificate validation.
