message: remove optimizer checking.
diff: @@ -25,7 +25,6 @@ from tensorflow.contrib.gan.python import train as tfgan_train
  from tensorflow.python.estimator import model_fn as model_fn_lib
  from tensorflow.python.estimator.canned import head
  from tensorflow.python.framework import ops
  -from tensorflow.python.training import optimizer
  __all__ = [
  'GANHead',
  @@ -96,10 +95,6 @@ class GANHead(head._Head): # pylint: disable=protected-access
  raise TypeError('generator_loss_fn must be callable.')
  if not callable(discriminator_loss_fn):
  raise TypeError('discriminator_loss_fn must be callable.')
  - if not isinstance(generator_optimizer, optimizer.Optimizer):
  - raise TypeError('generator_optimizer must be Optimizer.')
  - if not isinstance(discriminator_optimizer, optimizer.Optimizer):
  - raise TypeError('discriminator_optimizer must be Optimizer.')
  if not use_loss_summaries in [True, False, None]:
  raise ValueError('use_loss_summaries must be True, False or None.')
  if get_hooks_fn is not None and not callable(get_hooks_fn):
  