Should graduate students who are graders help undergraduate students with homework?
This is really a question for the professor(s) in charge of the course. You can and should get specific direction from them. 

However, just for background and context. A lone professor in a smaller class will find him/herself in this situation frequently. It is unlikely that it is wise to not answer at all, but also unwise to give such specific hints that student learning ceases and the student just follows directions. 

But it may take some experience to be able to balance the needs (and also to keep the prof happy). In general, you can think about the question you were asked, and before answering it, ask yourself whether the student has an unmet need that generates that question. Do they have a misconception? Have they missed an important reading? Something. I suspect that it is generally fair to address those needs rather than "helping" them with the specific assignment. Even giving them an easier task that might help them understand where they are and why they are having problems might be suitable. 

The base line is, don't get in the way of their learning just to help them complete a specific task. 

But you might check even this advice with the prof. You might also cause more specific training to be provided to you are your fellow graders/TAs.