small update for checkpoint evaluation

[dongwang218]

Why ?

make job print out more informative results by allow users to give each task a name.

How ?

• add task_id in job definition, and the task_id will be used to print out message. so instead of task-1, it could be gpqa_seed14.
• allow PYTHONPATH environment when calling job's script, this is needed when running external evaluation command.
• don't drop the assistant message if it is not empty so that we can force

Test plan

run checkpoint eval and observe the results.