pytorch中zero_grad中detach_的作用

def zero_grad(self):
    """Clears the gradients of all optimized :class:`Variable` s."""
    for group in self.param_groups:
        for p in group['params']:
            if p.grad is not None:
                p.grad.detach_()
                p.grad.zero_()

optimizer.zero_grad()中的p.grad.detach_()怎样理解，为什么梯度也需要detach_()