Create input tensor.

def create_input(self, t_input=None, forget_xy_shape=True):
    """Create input tensor."""
    if t_input is None:
      t_input = tf.placeholder(tf.float32, self.image_shape)
    t_prep_input = t_input
    if len(t_prep_input.shape) == 3:
      t_prep_input = tf.expand_dims(t_prep_input, 0)
    if forget_xy_shape:
      t_prep_input = model_util.forget_xy(t_prep_input)
    if hasattr(self, "is_BGR") and self.is_BGR is True:
      t_prep_input = tf.reverse(t_prep_input, [-1])
    lo, hi = self.image_value_range
    t_prep_input = lo + t_prep_input * (hi - lo)
    return t_input, t_prep_input