<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def prepare_encoder(inputs, hparams, attention_type="local_1d"):
 """Prepare encoder for images."""

  x = prepare_image(inputs, hparams, name="enc_channels")
  # Add position signals.
  x = add_pos_signals(x, hparams, "enc_pos")
  x_shape = common_layers.shape_list(x)
  if attention_type == "local_1d":
    x = tf.reshape(x, [x_shape[0], x_shape[1]*x_shape[2], hparams.hidden_size])
    x.set_shape([None, None, hparams.hidden_size])
  elif attention_type == "local_2d":
    x.set_shape([None, None, None, hparams.hidden_size])
  return x