Add save_conflict_info function & gen_corner_case examples

[xjsong99]

1. Add save_conflict_info function in NADE( ) class.
2. Add the return of conflict_vehicle_info in get_environment_maneuver_challenge( ) function.
3. Add script examples: gen_case_NADE.py, gen_case_NADE_with_av.py
4. Add script example: run_vis_gen.sh (batch generate visulization for generated corner cased by gen_case_NADE.py)
5. Add args in script/visualize_fcd.py