Random dag generator

There is a bug in the random_dag function of

https://ipython.org/ipython-doc/3/parallel/dag_dependencies.html

The solution is to add the following line:

if G.has_edge(a, b):continue

before:

G.add_edge(a,b)

Start with the random_dag function described in the linked DAG dependencies documentation and inspect how it adds edges. Reproduce or trace the duplicate-edge case, then verify that the generator no longer attempts to add an existing edge and that its resulting DAG behavior remains valid.

索引モデルが issue の本文から書いたものです。