message: Raise error if num_shards > 8 and per_host == True
  Currently, the per_host_input_for_training=True configuration only works for
  num_shards <= 8. In order to catch performance issues sooner, add a check to
  warn users if they are about to fall off a performance cliff. Future work
  will raise this restriction.
diff: @@ -1264,6 +1264,12 @@ class TPUEstimator(estimator_lib.Estimator):
  'eval batch size {} must be divisible by number of shards {}'
  .format(eval_batch_size, config.tpu_config.num_shards))
  + if (config.tpu_config.num_shards > 8 and
  + config.tpu_config.per_host_input_for_training):
  + # TODO(b/67051042): Support per_host input pipelines when num_shards > 8
  + raise NotImplementedError(
  + 'Per-host input pipelines only available for num_shards <= 8')
  +
  # Verifies the model_fn signature according to Estimator framework.
  estimator_lib._verify_model_fn_args(model_fn, params) # pylint: disable=protected-access
  # We cannot store config and params in this constructor as parent
  