Define a L1 regularizer.

  Args:
    weight: scale the loss by this factor.
    scope: Optional scope for name_scope.

  Returns:
    a regularizer function.

def l1_regularizer(weight=1.0, scope=None):
  """Define a L1 regularizer.

  Args:
    weight: scale the loss by this factor.
    scope: Optional scope for name_scope.

  Returns:
    a regularizer function.
  """
  def regularizer(tensor):
    with tf.name_scope(scope, 'L1Regularizer', [tensor]):
      l1_weight = tf.convert_to_tensor(weight,
                                       dtype=tensor.dtype.base_dtype,
                                       name='weight')
      return tf.multiply(l1_weight, tf.reduce_sum(tf.abs(tensor)), name='value')
  return regularizer