message: Loosen return type for variables.
diff: @@ -199,23 +199,23 @@ class Module(six.with_metaclass(ModuleMetaclass, tracking.AutoCheckpointable)):
  @property
  def variables(self):
  - """Returns a tuple of variables owned by this module and it's submodules.
  + """Collection of variables owned by this module and it's submodules.
  Returns:
  - A tuple of variables for the current module (sorted by attribute name)
  - followed by variables from all submodules recursively (depth first).
  + A collection of variables for the current module (sorted by attribute
  + name) followed by variables from all submodules recursively (depth first).
  """
  return tuple(walk(self, recurse_if=_IS_MODULE, predicate=_IS_VARIABLE))
  @property
  def owned_variables(self):
  - """Returns a tuple of variables that are attributes of the current module.
  + """Collection of variables that are attributes of the current module.
  See `variables` for a property which returns all variables from the current
  module and all it's submodules recursively.
  Returns:
  - A tuple of variables which are attributes of the current module. Will
  + A collection of variables which are attributes of the current module. Will
  yield variables inside nested structures (lists etc) but not in other
  modules.
  """
  @@ -223,24 +223,24 @@ class Module(six.with_metaclass(ModuleMetaclass, tracking.AutoCheckpointable)):
  @property
  def trainable_variables(self):
  - """Returns a tuple of variables owned by this module and it's submodules.
  + """Collection of variables owned by this module and it's submodules.
  Returns:
  - A tuple of variables for the current module (sorted by attribute name)
  - followed by variables from all submodules recursively (depth first).
  + A collection of variables for the current module (sorted by attribute
  + name) followed by variables from all submodules recursively (depth first).
  """
  return tuple(
  walk(self, recurse_if=_IS_MODULE, predicate=_IS_TRAINABLE_VARIABLE))
  @property
  def owned_trainable_variables(self):
  - """Returns a tuple of variables that are attributes of the current module.
  + """Collection of variables that are attributes of the current module.
  See `variables` for a property which returns all variables from the current
  module and all it's submodules recursively.
  Returns:
  - A tuple of variables which are attributes of the current module. Will
  + A collection of variables which are attributes of the current module. Will
  yield variables inside nested structures (lists etc) but not in other
  modules.
  """
  