FIX FSDP plugin update for QLoRA (#36720)
The _fsdp_qlora_plugin_updates checks for LoraConfig but other PEFT
methods can also support quantized models, e.g. VeRA. Therefore, the
isinstance check is now looking for PeftConfig in general.
Moreover, the fsdp_plugin variable may be undefined in the 2nd if
condition, leading to an `UnboundLocalError` error. This is fixed by not
assigning the variable at all.
I checked for tests that may need updating but only found
test_fsdp_config_transformers_auto_wrap associated with this change.
AFAICT, this test does not cover the changed code, since the test does
not start the training loop. Therefore, I haven't updated any tests. LMK
if/how this fix should be tested.
Co-authored-by: Marc Sun <57196510+SunMarc@users.noreply.github.com>
