[graph_trainer] Chunked CE loss doesn't work in graph trainer

Chunked CE loss was landed in #2937 and turned on by default. However, graph trainer hit tracing issues (https://github.com/pytorch/torchtitan/actions/runs/25014595498/job/73258965789).                                                                            
 
We disabled chunked CE loss in graph trainer in #3115, but it should be supported and turned on.  