如何将CNTK模型保存为文本？

import cntk as C

# 定义模型结构
input_dim = 784
output_dim = 10
hidden_dim = 200

input_var = C.input_variable(input_dim)
label_var = C.input_variable(output_dim)

# 定义模型参数
weights = C.parameter(shape=(input_dim, hidden_dim))
biases = C.parameter(shape=(hidden_dim))
output_weights = C.parameter(shape=(hidden_dim, output_dim))
output_biases = C.parameter(shape=(output_dim))

# 定义模型
hidden_layer = C.relu(C.times(input_var, weights) + biases)
output_layer = C.times(hidden_layer, output_weights) + output_biases

# 保存模型为文本
C.save_model(output_layer, "model.txt")