class_docstr: A dataset in mixed mode
code: def read(self):
      n = np.random.randint(3, 8)
      self.a = sp.csr_matrix(np.random.randint(0, 2, (n, n)))
      return [
          Graph(
              x=np.random.rand(n, f),
              e=np.random.rand(n, n, s),
              y=np.array([0.0, 1.0]),
          )
          for _ in range(n_graphs)
      ]
  
imports: import numpy
  import scipy.sparse
  from spektral.data import BatchLoader, DisjointLoader
  from spektral.data.dataset import Dataset
  from spektral.data.graph import Graph
  from spektral.data.loaders import MixedLoader, PackedBatchLoader, SingleLoader
label_desc: Use image node_id 1 for calling the TestDatasetMixed obj's underlying member method code with example usage: obj.read() and returns: unknown