[Pack] Random Number Generator in Packer is Non-Deterministic

[AlexandreSinger]

Within the GreedyCandidateSelector used in the packer, there is code which adds cluster molecule candidates by attraction groups. In this code, when the number of available atoms in the attraction group is larger than some threshold (currently 500), it randomly selects candidates from this group:

vtr-verilog-to-routing/vpr/src/pack/greedy_candidate_selector.cpp

Lines 785 to 807 in e46e9c2

	for (int j = 0; j < attraction_group_num_atoms_threshold_; j++) {
	// FIXME: This is a non-deterministic random number generator and it is
	// overkill to what this needs to be. Should use vtr::irand which
	// would be faster.
	std::random_device rd;
	std::mt19937 gen(rd());
	std::uniform_int_distribution<> distr(min, max);
	int selected_atom = distr(gen);
	AtomBlockId blk_id = available_atoms[selected_atom];
	//Only consider molecules that are unpacked and of the correct type
	t_pack_molecule* molecule = prepacker.get_atom_molecule(blk_id);
	if (!cluster_legalizer.is_mol_clustered(molecule) &&
	cluster_legalizer.is_molecule_compatible(molecule, legalization_cluster_id)) {
	add_molecule_to_pb_stats_candidates(molecule,
	cluster_gain_stats,
	packer_opts_.feasible_block_array_size,
	attraction_groups,
	atom_netlist_);
	}
	}
	}

The issue is that the random number generator used here is way overkill for this application and is non-deterministic. We should be using vtr::irand instead which would be faster.