It is not present in the doc as it is more of an implementation detail than something the user should rely on. We provide it for convenience and debugging purposes.
The tuple contains the next Nodes for each input. Each element there is a pair of net Node and the index in the output of the Tensor that was used as input to this Node.
the AccumulateGrad Nodes are just one type of Node. You can check the string representation and check that it matches I guess.
To collect these, you will need to traverse the whole graph (it is guaranteed that there is no cycle there) and save all the AccumulateGrad Nodes you encounter.