Setting ckpt_path='last' in Trainer.fit does not work with cloud checkpoints #18261

Bug description

Setting ckpt_path='last' in Trainer.fit will not work with cloud checkpoints (GCS, S3). This is because the current logic for finding checkpoint paths calls os.path.normpath which drops the gs:// and s3:// path prefixes for cloud paths, causing them to be treated as local paths later on.

https://github.com/Lightning-AI/lightning/blob/8f81dafd9512af8b4e42eeb30badc604883a92c4/src/lightning/pytorch/callbacks/model_checkpoint.py#L611-L620

What version are you seeing the problem on?

master

How to reproduce the bug

No response

Error messages and logs

# Error messages and logs here please

Environment

#- Lightning Component (e.g. Trainer, LightningModule, LightningApp, LightningWork, LightningFlow):
#- PyTorch Lightning Version (e.g., 1.5.0):
#- Lightning App Version (e.g., 0.5.2):
#- PyTorch Version (e.g., 2.0):
#- Python version (e.g., 3.9):
#- OS (e.g., Linux):
#- CUDA/cuDNN version:
#- GPU models and configuration:
#- How you installed Lightning(`conda`, `pip`, source):
#- Running environment of LightningApp (e.g. local, cloud):

More info

No response