Bump 3rdparty/Megatron-LM from fb7c3f8 to 6a6cd47

Bumps [3rdparty/Megatron-LM](https://github.com/NVIDIA/Megatron-LM) from `fb7c3f8` to `6a6cd47`.
- [Release notes](https://github.com/NVIDIA/Megatron-LM/releases)
- [Commits](NVIDIA/Megatron-LM@fb7c3f8...6a6cd47)

---
updated-dependencies:
- dependency-name: 3rdparty/Megatron-LM
  dependency-version: 6a6cd478839d90cf09a837adf8c79cbc844bc920
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]