add __all__=[] to python files not in API public list; import * only support in API public list files (#32643)

* update 2.0 public api in nn

* replace Chinese character cause error in ci;synchronization with pr:#32588 to avoid 'ascii' codec in python2

* numbers used in paddle.nn.functional.norm but not imported

* upgrade comment string to raw string

* fix string in

* fix string with ' '

* revert update on comments

* upgrade only necessary

* fix sample code checker

* fix comments with '''

add paddle.nn.initializer API, including: Normal, TruncatedNormal, Uniform, XavierNormal, XavierUniform, Assign (#27769)