Fix "amsgrad" is used before being defined when initializing the Adam…

…W optimizer (#660)

* Fix delayvar not correct in concat mode

* Update AdamW in optimizer.py

Fixed "amsgrad" is used before being defined.

brainpy/_src/optimizers/optimizer.py

@@ -901,6 +901,7 @@ def __init__(
       amsgrad: bool = False,
       name: Optional[str] = None,
   ):
+    self.amsgrad = amsgrad
     super(AdamW, self).__init__(lr=lr,
                                 train_vars=train_vars,
                                 weight_decay=weight_decay,
@@ -919,7 +920,6 @@ def __init__(
     self.beta2 = beta2
     self.eps = eps
     self.weight_decay = weight_decay
-    self.amsgrad = amsgrad
 
   def __repr__(self):
     return (f"{self.__class__.__name__}(lr={self.lr}, "