[docs] Pin removed Megatron checkpoint script link

[pratikgx]

What does this PR do?

The Megatron-LM guide links to checkpoint_reshaping_and_interoperability.py on transformers main, but that file was removed in huggingface/transformers 54b89a253d (#47764), so the link 404s.

• docs/source/usage_guides/megatron_lm.md: transformers/blob/main/src/transformers/models/megatron_gpt2/checkpoint_reshaping_and_interoperability.py -> same path at tag v5.14.1, the last release that still has the script.

Docs only; the new target was checked at that tag.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

Who can review?

Documentation: @SunMarc

[SunMarc]

we don't update megatron doc or code anymore