Rename corpus reference to train_corpus in blackbox_learner.py

This patch renames the corpus refereince in blackbox_learner.py from
sampler to train_corpus to better represent what it actually is. This
patch also fixes an incorrect string as a byproduct of this where an
argument name in the docstring did not match the actual argument name.

compiler_opt/es/blackbox_learner.py

@@ -128,7 +128,7 @@ class BlackboxLearner:
 
  def __init__(self,
  blackbox_opt: blackbox_optimizers.BlackboxOptimizer,
- sampler: corpus.Corpus,
+ train_corpus: corpus.Corpus,
  tf_policy_path: str,
  output_dir: str,
  policy_saver_fn: PolicySaverCallableType,
@@ -141,7 +141,7 @@ def __init__(self,
 
  Args:
  blackbox_opt: the blackbox optimizer to use
- train_sampler: corpus_sampler for training data.
+ train_corpus: the training corpus to utiilize
  tf_policy_path: where to write the tf policy
  output_dir: the directory to write all outputs
  policy_saver_fn: function to save a policy to cns
@@ -152,7 +152,7 @@ def __init__(self,
  deadline: the deadline in seconds for requests to the inlining server.
  """
  self._blackbox_opt = blackbox_opt
- self._sampler = sampler
+ self._train_corpus = train_corpus
  self._tf_policy_path = tf_policy_path
  self._output_dir = output_dir
  self._policy_saver_fn = policy_saver_fn
@@ -250,7 +250,8 @@ def _get_results(
  perturbations: List[bytes]) -> List[concurrent.futures.Future]:
  if not self._samples:
  for _ in range(self._config.total_num_perturbations):
- sample = self._sampler.sample(self._config.num_ir_repeats_within_worker)
+ sample = self._train_corpus.sample(
+ self._config.num_ir_repeats_within_worker)
  self._samples.append(sample)
  # add copy of sample for antithetic perturbation pair
  if self._config.est_type == (