[Bug]: edge_gpt_free: broken image generation

Describe the bug | 简述

After https://github.com/binary-husky/gpt_academic/commit/cd411c2eeae1180b3880da1d6fe8c6c341ed60f3 ImageGen and ImageGenAsync classes were removed, but they are still used in the code for image generation:

https://github.com/binary-husky/gpt_academic/blob/d6bde0fa54373309bd05823a49bda8da019d2c77/request_llms/edge_gpt_free.py#L543-L546

https://github.com/binary-husky/gpt_academic/blob/d6bde0fa54373309bd05823a49bda8da019d2c77/request_llms/edge_gpt_free.py#L1032-L1036