`save_params()` and `load_params()` in `fluid.io` are unavailable

[JiayiFeng]

save_params() and load_params() are used to implement model saving and loading. The model is supposed to be able to continue its training after a saving and loading. However, using save_params() and load_params() cannot satisfy the needs.

The save_params() only save Parameters. But in the Program there are some other critical Variables who are not Parameter, such as optimizers' learning rates and momentums.

The load_params() takes a serialized ProgramDesc protobuf object as its input, and load all Parameters in it. However, there is no such concept of Parameter in the protobuf. All Parameters degrade to normal Variables during flushing into protobuf. Therefore, nothing will be loaded by load_params().

Using save_persistables() and load_persistables_if_exist() seems an avaiable but ungainly substitution.

[shanyi15]

您好，此issue在近一个月内暂无更新，我们将于今天内关闭。若在关闭后您仍需跟进提问，可重新开启此问题，我们将在24小时内回复您。因关闭带来的不便我们深表歉意，请您谅解~感谢您对PaddlePaddle的支持!
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