'Get gradient op_trees w.r.t the list of `tensors`. If a tensor is not
used, its gradient will be set to zero.
Arguments:
Tensors (list): List of Tensors to compute gradients.
Returns
list: A list of op_trees, each of them is the gradent of the input
tensor.'

def get_grad_op_tree(self, tensors):

    grad_op_trees = []
    for tensor in tensors:
        grad_op_trees.append(self.map_tensor_grad_op_tree.get(tensor._original_base, (tensor * 0.0)))
    return grad_op_trees
