deepspeed-chat: fix bf16 stage2 accuracy for bloom-560m #772
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Bloom-560m model has high variance in its last LN layer weight. This causes accuracy issues in bf16 stage2 training. Therefore, reset the parameters of the last LN layer before training. This is a good practice in any case where we replace the classifier that follows the LN.
In addition, in case we are using only optimize lora, we need to force the training of the LN parameters that were reset.
Note that current fix uses plain initialization of final LN. A separate commit will provide support for zero3 initialization.
Change-Id: I323d8947907eb4a1cc0fa6354bdaf0cbbf33a68d