chore(beep boop 🤖): Bump `MCORE_TAG=71c394b...` (2024-12-16)
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🚀 PR to bump `NVIDIA/Megatron-LM` in `Dockerfile.ci` to `MCORE_TAG=71c394b172ce4f1b9466e1d728bad5cc6314d15d`.  

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

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