Arbitrary file read via shard path traversal

Path traversal in checkpoint index handling in Hugging Face Transformers 5.15.0 and earlier allows attackers to read local files via a malicious model directory. In get_checkpoint_shard_files, shard names from the index weight_map are passed to os.path.join without rejecting .. segments or absolute paths, so the computed shard path can escape the model directory. Exploitation requires a user or automated workflow to call from_pretrained on an untrusted local sharded checkpoint; the demonstrated impact is file disclosure and filesystem reconnaissance rather than code execution.