code: def test_flatten_dict(self):
      xs = {"foo": 1, "bar": {"a": 2, "b": {}}}
      flat_xs = traverse_util.flatten_dict(xs)
      self.assertEqual(
          flat_xs,
          {
              ("foo",): 1,
              ("bar", "a"): 2,
          },
      )
      flat_xs = traverse_util.flatten_dict(freeze(xs))
      self.assertEqual(
          flat_xs,
          {
              ("foo",): 1,
              ("bar", "a"): 2,
          },
      )
      flat_xs = traverse_util.flatten_dict(xs, sep="/")
      self.assertEqual(
          flat_xs,
          {
              "foo": 1,
              "bar/a": 2,
          },
      )
  
imports: import collections
  import jax
  import jax.numpy
  import numpy
  import optax
  from absl.testing import absltest
  import flax
  from flax import traverse_util
  from flax.core import freeze
label_desc: Use image node_id 15 for calling the TraversalTest obj's underlying member method code with example usage: obj.test_flatten_dict() without return types