Save exclude_from_weight_decay in config for LAMB (#2619)
Save exclude_from_weight_decay in config for LAMB (#2619)

Before:
```py
import tensorflow_addons as tfa
mm = keras.models.Sequential([keras.layers.Input([32, 32, 3]), keras.layers.Flatten(), keras.layers.Dense(10)])
mm.compile(optimizer=tfa.optimizers.LAMB(learning_rate=0.1, weight_decay_rate=0.02, exclude_from_weight_decay=['/gamma', '/beta']))
mm.save('aa.h5')
bb = keras.models.load_model('aa.h5')
print(bb.optimizer.exclude_from_weight_decay)
# None, None
```
After:
```py
import tensorflow_addons as tfa
mm = keras.models.Sequential([keras.layers.Input([32, 32, 3]), keras.layers.Flatten(), keras.layers.Dense(10)])
mm.compile(optimizer=tfa.optimizers.LAMB(learning_rate=0.1, weight_decay_rate=0.02, exclude_from_weight_decay=['/gamma', '/beta']))
mm.save('aa.h5')
bb = keras.models.load_model('aa.h5')
print(bb.optimizer.exclude_from_weight_decay, bb.optimizer.exclude_from_layer_adaptation)
# ['/gamma', '/beta'] ['/gamma', '/beta']
```
tensorflow/addons,tensorflow/addons,tensorflow/addons