Your callback function should look something like this:  
```python
def callback_dynamic_cfg(pipe, step_index, timestep, callback_kwargs):
# adjust the batch_size of prompt_embeds according to guidance_scale
if step_index == int(pipeline.num_timesteps * 0.4):
prompt_embeds = callback_kwargs["prompt_embeds"]
prompt_embeds = prompt_embeds.chunk(2)[-1]