message: Disallow mixed precision to be used with TPUStrategy.
  This is untested and probably very buggy. AutoCastVariables are known not to work properly with TPUMirroredVariables in certain cases.
diff: @@ -1688,6 +1688,14 @@ class Layer(module.Module):
  else:
  self._dtype_policy = policy.global_policy()
  + if self._dtype_policy.should_cast_variables and backend.is_tpu_strategy(
  + ds_context.get_strategy()):
  + # TODO(b/137859335): Supoprt this. AutoCastVariables currently do not work
  + # properly when wrapping TPUMirroredVariables.
  + raise ValueError('DType Policies ending in "_with_float32_vars" are '
  + 'not yet supported with TPUStrategy. Got policy: %s' %
  + self._dtype_policy.name)
  +
  # This has no impact on the layer behavior, and is only used for printing
  # warnings.
  self._dtype_defaulted_to_floatx = (not dtype and
  