code: def build_dense_graph(n_particles):
      g = nx.complete_graph(n_particles)
      return dgl.from_networkx(g)
  
imports: import copy
  import os
  import dgl
  import networkx
  import numpy
  import torch
  from torch.utils.data import DataLoader, Dataset
label_desc: Use image node_id 1 for calling a global function with example usage: build_dense_graph(n_particles) and returns: dgl