message: Fix: Ops backend to run on PlaidML
  Previously, Ops backend was running on Tensorflow
  backend despite the backend being set to `plaid`.
  This patch fixes this issue.
diff: @@ -130,7 +130,8 @@ class Frontend(core.Frontend):
  if backend == 'plaid':
  try:
  self.configuration['plaid'] = importlib.import_module('plaidml').__version__
  - importlib.import_module('plaidml.bridge.keras')
  + os.environ['KERAS_BACKEND'] = 'plaidml.bridge.keras'
  + importlib.import_module('keras.backend')
  except ImportError:
  raise core.ExtrasNeeded(['plaidml-keras'])
  elif backend == 'tc':
  