@TRAN_KHANH1, to add to @gautamaltman’s comments:
I can think of one reason you might want to choose to use detach(). Since detach() creates a new tensor, if you use fake.detach() for the discriminator, you could theoretically still use the same fake with your generator where you want the gradients. I think some of the assignments take advantage of this.
I suspect the course developers used detach() in all assignments for consistency to help students focus on the main concepts, but you’re absolutely right that as long as you don’t need/want to reuse fake, the requires_grad_ approach is more efficient.