fix load pretrain and load params due to the shape check in fluid.io

[SunGaofeng]

Fix load pretrain and load params due to the shape check in fluid.io added in Version 1.6.0 In StNet and Nonlocal, some of the pretrained params' shape is not the same as those in the program, this will lead to error in Version 1.6.0. This pr modified the loading process by creating vars in another scope and then load pertained params. When loading finished, the value will be transformed as set to the parameters of the target program.

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