[`PEFT`] Update the example scripts to reflect PEFT API

[younesbelkada]

PEFT will be soon used as a backend for LoRA related logic !

Let's make sure to reflect the new API in the example scripts, e.g. change the monkey patch calls to add_adapter

Related: #5151

[younesbelkada]

#5388 is still open, we will merge it right after the next PEFT release