input: Write a function to generate a natural language response for a given utterance using a pre-trained TensorFlow Keras model. The function should take an utterance as input and return the predicted response. If the model is not trained or the input is invalid, the function should raise an appropriate exception.
  assert generate_response("What time is it?")==prediction
  assert generate_response("How are you?")==prediction
  assert generate_response("What is the weather today?")==prediction
  assert generate_response("")==Exception("Invalid input")
  assert generate_response(12345)==Exception("Invalid input")
output: def generate_response(utterance):
      if not isinstance(utterance, str) or not utterance.strip():
          raise Exception("Invalid input")
      # Assuming the model is already loaded and named 'model'
      prediction = model.predict(utterance)
      return prediction