message: Test is_placeholder
diff: @@ -1257,6 +1257,14 @@ class TestBackendOps(unittest.TestCase):
  a = b.constant(5, shape=(10,))
  return a
  + def testIsPlaceholder(self):
  + x = pkb.placeholder((4, 3))
  + self.assertTrue(pkb.is_placeholder(x))
  + y = pkb.variable(m(2, 2))
  + self.assertFalse(pkb.is_placeholder(y))
  + z = pkb.exp(x)
  + self.assertFalse(pkb.is_placeholder(z))
  +
  # Note: we skip tensorflow since init_global must be called in the middle of this function
  # for correct semantics, and Theano is sufficient.
  @compareForwardExact(skip_tensorflow=True)
  