Bump 3rdparty/Megatron-LM from c76410a to 4fb4c3d (#624)

Bumps [3rdparty/Megatron-LM](https://github.com/NVIDIA/Megatron-LM) from
`c76410a` to `4fb4c3d`.
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/NVIDIA/Megatron-LM/commit/4fb4c3dc61b26784ccf804b340ba047bbcec3953"><code>4fb4c3d</code></a>
Merge branch 'ko3n1g/chore/bump-versions' into 'main'</li>
<li><a
href="https://github.com/NVIDIA/Megatron-LM/commit/9c11ab4ca24ead28c3c1e29f8904f8258d7543cb"><code>9c11ab4</code></a>
ADLR/megatron-lm!2510 - chore: Bump versions</li>
<li><a
href="https://github.com/NVIDIA/Megatron-LM/commit/7ba0d6ddaab1e4d1c587af8fea8a0dfe13b20f13"><code>7ba0d6d</code></a>
Merge branch 'ko3n1g/ci/ci-output' into 'main'</li>
<li><a
href="https://github.com/NVIDIA/Megatron-LM/commit/57c392be433263ac94095f024237827eccdb5ae0"><code>57c392b</code></a>
ADLR/megatron-lm!2571 - ci: Better output</li>
<li><a
href="https://github.com/NVIDIA/Megatron-LM/commit/c7bf403ba37ce7ca739a025accac232c75a38966"><code>c7bf403</code></a>
Merge branch 'dpykhtar/fix_load_ckpt' into 'main'</li>
<li><a
href="https://github.com/NVIDIA/Megatron-LM/commit/37a900f021ea2c9a15da096955d1411d41bb07f4"><code>37a900f</code></a>
ADLR/megatron-lm!2555 - set weight_only to False</li>
<li><a
href="https://github.com/NVIDIA/Megatron-LM/commit/fa352269f672785234bd399eda5758999cde3883"><code>fa35226</code></a>
Merge branch 'ko3n1g/refactor/make-get_mlp_module_spec-public' into
'main'</li>
<li><a
href="https://github.com/NVIDIA/Megatron-LM/commit/4e87b4c06da7a11751d51119e55d07027156e0b8"><code>4e87b4c</code></a>
ADLR/megatron-lm!2534 - refactor: Make <code>get_mlp_module_spec</code>
public</li>
<li><a
href="https://github.com/NVIDIA/Megatron-LM/commit/e02a860cda6459222ae109c76fd7c5e9c27d45fd"><code>e02a860</code></a>
Merge branch 'ko3n1g/ci/fetch-exitcode' into 'main'</li>
<li><a
href="https://github.com/NVIDIA/Megatron-LM/commit/f85b6b1f6093f75e2062e07a8bbd2c6b98b40646"><code>f85b6b1</code></a>
ADLR/megatron-lm!2562 - ci: Fetch exit-code</li>
<li>Additional commits viewable in <a
href="https://github.com/NVIDIA/Megatron-LM/compare/c76410adec0eb1f81544344fcc2c404166e9913d...4fb4c3dc61b26784ccf804b340ba047bbcec3953">compare
view</a></li>
</ul>
</details>
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