message: Avoid redundant isinstance checks for performance
  Improves the following micro benchmarks (examples per sec):
  benchmarkFunctionWithFiveHundredResourceInputs: 163 -> 183
  benchmarkFunctionWithFiveResourceInputs: 7671 -> 8140
diff: @@ -1540,10 +1540,6 @@ class ConcreteFunction(object):
  if default_graph.building_function and not self._func_graph.saveable:
  default_graph.mark_as_unsaveable(self._func_graph.saving_errors)
  - if any(isinstance(a, composite_tensor.CompositeTensor) for a in args):
  - raise AssertionError("Expected all args to be Tensors or Variables; "
  - "but got CompositeTensor: %r" % args)
  -
  if (tape.could_possibly_record() or
  hasattr(ops.get_default_graph(), "watch_variable")):
  for v in self._func_graph.variables:
  