message: Fix docstring to match implementation of
  tf.config.experimental.get_virtual_device_configuration
diff: @@ -466,11 +466,15 @@ def get_virtual_device_configuration(device):
  ```python
  physical_devices = tf.config.experimental.list_physical_devices('CPU')
  assert len(physical_devices) == 1, "No CPUs found"
  + configs = tf.config.experimental.get_virtual_device_configuration(
  + physical_devices[0])
  + assert configs is None
  tf.config.experimental.set_virtual_device_configuration(
  physical_devices[0],
  [tf.config.experimental.VirtualDeviceConfiguration(),
  tf.config.experimental.VirtualDeviceConfiguration()])
  - configs = tf.config.experimental.get_virtual_device_configuration('CPU')
  + configs = tf.config.experimental.get_virtual_device_configuration(
  + physical_devices[0])
  assert len(configs) == 2
  ```
  @@ -478,7 +482,9 @@ def get_virtual_device_configuration(device):
  device: PhysicalDevice to query
  Returns:
  - List of VirtualDeviceConfiguration objects
  + List of `tf.config.experimental.VirtualDeviceConfiguration` objects or
  + `None` if no virtual device configuration has been set for this physical
  + device.
  """
  return context.context().get_virtual_device_configuration(device)
  