chore(beep boop 🤖): Bump `MCORE_TAG=726da58...` (2025-01-12)
🚀 PR to bump `NVIDIA/Megatron-LM` in `Dockerfile.ci` to `MCORE_TAG=726da588ead74ac7f5e62324dea478c1dabb4150`.  

📝 Please remember the following to-do's before merge: 
- [ ] Verify the presubmit CI  

🙏 Please merge this PR only if the CI workflow completed successfully.