message: Implement K.is_placeholder() in plaidml2 keras backend
diff: @@ -932,7 +932,9 @@ def is_keras_tensor(x):
  @_log_call
  def is_placeholder(x):
  - _report_unimplemented('is_placeholder')
  + if not is_tensor(x):
  + return False
  + return x.opname == 'placeholder'
  @_log_call
  