Usability improvement for tf.constant_initializer

* Raise TypeError if the input value is not one of the expected types,
  e.g., a tf.Tensor. Previously, the error is delayed until Session.run()
  time and the message is very obscure, e.g.,
  "TypeError: Expected float32, got list containing Tensors of type
   '_Message' instead."

PiperOrigin-RevId: 180416775