add attr() interface to layer

[jacquesqiao]

fix: #1811

import paddle.v2 as paddle x = paddle.layer.data(name='x', type=paddle.data_type.dense_vector(13)) y_predict = paddle.layer.fc(input=x, size=1, act=paddle.activation.Linear()) y = paddle.layer.data(name='y', type=paddle.data_type.dense_vector(1)) cost = paddle.layer.mse_cost(input=y_predict, label=y) print cost.attr() #print cost.attr().size

will get

name: "__mse_cost_0__" type: "square_error" size: 1 active_type: "" inputs { input_layer_name: "__fc_layer_0__" } inputs { input_layer_name: "y" } coeff: 1.0