code: def build_conv_model(self, model_type):
      if model_type == "GCN":
          return pyg_nn.GCNConv
      elif model_type == "GAT":
          return pyg_nn.GATConv
      elif model_type == "GraphSage":
          return pyg_nn.SAGEConv
      else:
          raise ValueError(f"Model {model_type} unavailable")
  
file_path: pytorch_geometric/graphgym/custom_graphgym/network/example.py
imports: import torch
  import torch.nn
  import torch.nn.functional
  import torch_geometric.graphgym.models.head
  import torch_geometric.graphgym.register
  import torch_geometric.nn
  from torch_geometric.graphgym.config import cfg
  from torch_geometric.graphgym.register import register_network
label_desc: Use image node_id 2 for calling the ExampleGNN obj's underlying member method code with example usage: obj.build_conv_model(model_type) and returns: pyg_nn, pyg_nn, pyg_nn