Fix PeftSavingCallback

[lchu6]

This PR introduces fix on PeftSavingCallback for both saving PEFT and non-PEFT (i.e. full parameter tuning).

1. Fix saving peft when original implementation would save peft in root ckpt folder which violates the purpose of this callback.
2. Fix saving full-parameter-tuning when it is actually not necessary for saving separately. And original implementation would introduce broken ckpt as discussed in model checkpoints are broken when doing full parameter tuning  #23

[Ssukriti]

@fabianlim does this PR become obsolete with #53 ?

[fabianlim]

@Ssukriti to my knowledge yes, as these checkpoint manipulation techniques are not required anymore, now that HF trainer will save the checkpoints correct as described in the writeup of #53

[Ssukriti]

closing unless asked otherwise. PR #53 will be merged soon