chore(beep boop 🤖): Bump `MCORE_TAG=6486203...` (2025-01-14)
🚀 PR to bump `NVIDIA/Megatron-LM` in `Dockerfile.ci` to `MCORE_TAG=64862035dbb9b03f40b09cfe26b82306609f7153`.  

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

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