[data] fix: log skipped VLM dataloader samples as warnings

[kaimo455]

Summary

• replace stderr printing and traceback dumping in the VLM dataloader skip handler with logging.warning
• keep Energon sample-skip behavior while producing concise warning logs with sample key and exception type

Adapted from internal iCode review 120857904 patchset 1.

Verification

• final GitHub blob matches iCode patchset 1 for loongforge/data/multimodal/base/task_encoder.py
• git diff --check origin/master..HEAD
• python -m compileall -q loongforge/data/multimodal/base/task_encoder.py