fix peft ckpts not being pushed to hub 
# What does this PR do?

1. Currently, when ckpts are saved every `save_steps` and `push_to_hub=True`, PEFT ckpts aren't being pushed to hub.
Reason:
every `save_steps` , the ckpts are being written to local `checkpoint-xx` folders. Now, in the trainer's function `_push_from_checkpoint` , it will copy the model files from the latest ckpt folder `checkpoint-xx` to the `output_dir`  which gets pushed to the Hub. Note that the modeling_files doesn't have `ADAPTER_WEIGHTS_NAME`  or `ADAPTER_SAFE_WEIGHTS_NAME`  or `ADAPTER_CONFIG_NAME`  leading to them not being pushed.

This PR fixes it. 

cc @younesbelkada 
_The documentation is not available anymore as the PR was closed or merged._