Algos which don't use internal PLC graph instance create a redundant graph

[rlratzel]

The updates to 22.08 needed to avoid creating a graph at algo run time result in a redundant graph being created for algos that don't use the internal PLC graph instance, which wastes memory.

We may need to make the internal PLC graph instance an option until all legacy algos are updated, and non-legacy algos that need a PLC graph instance will either use it if present or fall back to creating it at algo run time.

[rlratzel]

The current workaround to reclaim memory for legacy algos is to delete the internal PLC graph after constructing the Graph instance:

from cugraph.experimental import datasets
G = datasets.karate.get_graph()
del G._Impl._plc_graph