Fix "amsgrad" is used before being defined when initializing the AdamW optimizer

[CloudyDory]

When initializing the "AdamW" optimizer, property "amsgrad" is used before it is defined. Therefore the following code generates error: AttributeError: 'AdamW' object has no attribute 'amsgrad'.

import brainpy as bp
import brainpy.math as bm

weights = {'weight': bm.TrainVar(bm.zeros(10))}
optimizer = bp.optim.AdamW(lr=0.01, train_vars=weights)

This PR fixes this error.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• Code follows the code style of this project.
• Changes follow the CONTRIBUTING guidelines.
• Update necessary documentation accordingly.
• Lint and tests pass locally with the changes.
• Check issues and pull requests first. You don't want to duplicate effort.

[chaoming0625]

Thanks for the fixing.