[BUG] Drop duplicates edges before symmetrizing the Graph

[jnke2016]

Describe the bug Our current implementation doesn't drop the duplicated edges which can be ambiguous during the symmetrization step. In fact, we currently concat the src to dst and dst to src. This can double the number of multi edges if the user is creating a multi graph. Dropping the multi edge might not be a viable solution because those multi edges should be preserved if the user is creating a multi graph. Another relevant point related to the symmetrization call is to set the value of split_out to the initial number of partition of partition instead of the number of workers during group_by calls and then repartition it before persisting the dask object.