Return the BFS layers of a graph as a list of lists.
Parameters:
graph – The input graph to use. Can either be a
PyGraph or PyDiGraph
sources (list[int]) – An optional list of node indices to use as the starting
nodes for the BFS traversal. If not specified, all nodes in the graph
will be used as sources.
Returns:
A list of lists where each inner list contains the node indices
at that BFS layer/level from the source nodes