[fix] Add trailing slash to local directory for uploads to existing cloud directory with fsspec

[SumanthRH]

What does this PR do?

Fixes an issue with checkpoint upload format with LoRA. With LoRA, we make multiple calls to io.local_work_dir , which in turn calls io.upload_directory . Each time, we write checkpoint files to a temporary file and then upload them to directory in cloud storage. Currently, subsequent calls to io.upload_directory leads to nesting of ckpt contents:

 aws s3 ls <my_remote_path>/global_step_40/policy/
                           PRE tmp8uxmdhfx/
                           PRE tmpbbhsijnu/
                           PRE tmpd3xtpaoz/
                           PRE tmpeag11l_c/
                           PRE tmppg5zt9uh/
                           PRE tmpqplxqem5/
2026-05-01 00:01:40      14885 extra_state_world_size_8_rank_6.pt
2026-05-01 00:01:40      14885 extra_state_world_size_8_rank_7.pt
2026-05-01 00:01:40  860448401 model_world_size_8_rank_6.pt
2026-05-01 00:01:40  860448401 model_world_size_8_rank_7.pt
2026-05-01 00:01:40 1720907321 optim_world_size_8_rank_6.pt
2026-05-01 00:01:40 1720907321 optim_world_size_8_rank_7.pt

The root cause is that with fsspec, while uploading files in a directory src to an existing directory dst with fsspec, we need to ensure that the file path ends in a trailing slash. otherwise, fsspec will create a subdirectory dst/src instead of directly syncing contents of src into the root dst directory

[gemini-code-assist]

Code Review

This pull request modifies the upload_directory function in skyrl/backends/skyrl_train/utils/io/io.py to ensure the local path ends with a trailing slash, preventing fsspec from creating unwanted subdirectories during synchronization. A review comment points out that the manual string concatenation used is not cross-platform and suggests using os.path.join to ensure the trailing separator is handled correctly on all operating systems.