[recipe] feat: Add DeepSeek-V4 training smoke configs

[Meirtz]

What does this PR do ?

Adds minimal DeepSeek-V4 training smoke configs for exercising selected Megatron Core DSv4 architecture paths through Megatron Bridge. These configs are intended for short validation runs, not full production pretraining recipes.

Changelog

• Add deepseek_v4_tiny_pretrain_config for a 1-layer DSv4 hybrid-attention smoke path.
• Add deepseek_v4_flash_proxy_pretrain_config for DSv4 hybrid attention with mHC and hash-routed MoE.
• Add deepseek_v4_flash_mtp_proxy_pretrain_config for DSv4 hybrid attention with mHC and MTP.
• Export the new recipe helpers and add unit and functional smoke coverage.
• Pin the Megatron Core dev submodule to a commit that contains the required DSv4 primitives.

GitHub Actions CI

See the CI section in the Contributing doc for how to trigger the CI. A Nvidia developer will need to approve and trigger the CI for external contributors.

Local validation:

• uv run --no-sync --with pre-commit pre-commit run --all-files
• Focused GB200 recipe smoke validation for the DSv4 tiny, mHC/hash-MoE proxy, and mHC/MTP proxy paths.

Before your PR is "Ready for review"

Pre checks:

• Make sure you read and followed Contributor guidelines
• Did you write any new necessary tests?
• Did you add or update any necessary documentation? Not needed for these smoke-only recipe configs.
• Does the PR affect components that are optional to install? No.

Additional Information

This PR intentionally limits the scope to smoke configs and tests. It does not claim full DeepSeek-V4 training support.

The recipe configs currently depend on the Megatron Core dev pin because the required DSv4 primitives are not yet fully available in the Megatron Core main pin.

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