Implement a true `operator==` for `ParallelComputationGraph` and resolve the related hash issue

[wmdi]

We should provide a true operator== for ParallelComputationGraph, which means g1 == g2 iff they always gives the same outputs when given the same inputs. We allow the unavoidable performance issue in the comparison.

Also see https://reviewable.io/reviews/flexflow/FlexFlow/1229#-NpXFKHVCMVgHr2GOPAK