Potential boundary error in utils.py

Hi there, in the file [utils.py](https://github.com/InternLM/xtuner/blob/main/xtuner/model/utils.py), there might be an error involving accessing beyond the boundary on line 208 for the operation `cur_inputs_embeds_no_im[i]`, when the IMAGE_TOKEN_INDEX is located at the end of the `input_ids`. Please consider to solve it.