class_docstr: ZINC dataset for the graph regression task.
  
  A subset (12K) of ZINC molecular graphs (250K) dataset is used to
  regress a molecular property known as the constrained solubility.
  For each molecular graph, the node features are the types of heavy
  atoms, between which the edge features are the types of bonds.
  Each graph contains 9-37 nodes and 16-84 edges.
  
  Reference `<https://arxiv.org/pdf/2003.00982.pdf>`_
  
  Statistics:
  
  Train examples: 10,000
  Valid examples: 1,000
  Test examples: 1,000
  Average number of nodes: 23.16
  Average number of edges: 39.83
  Number of atom types: 28
  Number of bond types: 4
  
  Parameters
  ----------
  mode : str, optional
      Should be chosen from ["train", "valid", "test"]
      Default: "train".
  raw_dir : str
      Raw file directory to download/contains the input data directory.
      Default: "~/.dgl/".
  force_reload : bool
      Whether to reload the dataset.
      Default: False.
  verbose : bool
      Whether to print out progress information.
      Default: False.
  transform : callable, optional
      A transform that takes in a :class:`~dgl.DGLGraph` object and returns
      a transformed version. The :class:`~dgl.DGLGraph` object will be
      transformed before every access.
  
  Attributes
  ----------
  num_atom_types : int
      Number of atom types.
  num_bond_types : int
      Number of bond types.
  
  Examples
  ---------
  >>> from dgl.data import ZINCDataset
  
  >>> training_set = ZINCDataset(mode="train")
  >>> training_set.num_atom_types
  28
  >>> len(training_set)
  10000
  >>> graph, label = training_set[0]
  >>> graph
  Graph(num_nodes=29, num_edges=64,
      ndata_schemes={'feat': Scheme(shape=(), dtype=torch.int64)}
      edata_schemes={'feat': Scheme(shape=(), dtype=torch.int64)})
code: def num_atom_types(self):
      return 28
  
file_path: dgl/python/dgl/data/zinc.py
imports: import os
  from .dgl_dataset import DGLBuiltinDataset
  from .utils import _get_dgl_url, load_graphs
label_desc: Use image node_id 6 for calling the ZINCDataset obj's underlying member method code with example usage: obj.num_atom_types() and returns: int