Backend paddle: add optimizers with supporting regularizer

[lijialin03]

add sgd/rmsprop/adamw with supporting regularizer for paddle backend

[lijialin03]

A code style error occurred, "Access to a protected member _coeff of a client class (protected-access)".
It seems that weight_decay._coeff cannot be called directly.（The strange thing is that this error did not appear in the first few submissions）