add some frequently asked questions into F&Q

[lcy-seso]

We are going to select some frequently asked questions into F&Q. I will keep on updating this issue to keep trace of issues we selected.

• get outputs of multiple layers in inferring: 如何在paddle.v2中输出网络中多个中间层 #4153
• Notes of PaddlePaddle: Some notes about PaddlePaddle, hope someone can help me to refine it and add them into F&Q #3070
• load pre-trained parameters, formats of the saved parameters in PaddlePaddle. 怎么加载预训练的embedding层，（embedding里面的两个脚本怎么使用） #2778 PaddlePaddle存储的二进制模型参数文件的格式是什么样，如何转为明文？ #2812
• how to slove the gradient explosion A temporary guide to stablize models' training #2262
• duplicate layer error 模型训练没有问题，预测阶段出现Duplicated layer name 错误 #3615
• learning rate annealing: 如何设置learning rate decay？Annealing the learning rate  #1167 (comment)