message: Create BT input datasets within input_fn
  Outside of input_fn, they can get placed in a separate TF graph,
  causing an error at runtime.
diff: @@ -140,14 +140,13 @@ def train(*tf_records: "Records to train on"):
  if FLAGS.use_tpu:
  effective_batch_size *= FLAGS.num_tpu_cores
  + if FLAGS.use_tpu:
  if FLAGS.use_bt:
  + def _input_fn(params):
  games = bigtable_input.GameQueue(
  FLAGS.cbt_project, FLAGS.cbt_instance, FLAGS.cbt_table)
  games_nr = bigtable_input.GameQueue(
  FLAGS.cbt_project, FLAGS.cbt_instance, FLAGS.cbt_table + '-nr')
  - if FLAGS.use_tpu:
  - if FLAGS.use_bt:
  - def _input_fn(params):
  return preprocessing.get_tpu_bt_input_tensors(
  games,
  games_nr,
  