Update freezable_sync_batch_norm.py

[ZoeyZiyunWang]

This code is not compatible with the newest version of Tensorflow (2.16.1). Therefore I Replace the line: "class FreezableSyncBatchNorm(tf.keras.layers.experimental.SyncBatchNormalization):" With:
"class FreezableSyncBatchNorm(tf.keras.layers.BatchNormalization):" to make it compatible with the newest Tenserflow version (2.16.1).

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