Bump 3rdparty/Megatron-LM from f8d3e2e to 188435a

Bumps [3rdparty/Megatron-LM](https://github.com/NVIDIA/Megatron-LM) from `f8d3e2e` to `188435a`.
- [Release notes](https://github.com/NVIDIA/Megatron-LM/releases)
- [Commits](NVIDIA/Megatron-LM@f8d3e2e...188435a)

---
updated-dependencies:
- dependency-name: 3rdparty/Megatron-LM
  dependency-version: 188435a1d00a7ed29fdd169a17a36f75b496a558
  dependency-type: direct:production
...

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Author: dependabot[bot]