[guihong-nv]

• I, the PR author, have personally reviewed every line of this PR.

What does this PR do ?

Makes --position-embedding-type yarn actually usable for hybrid models by declaring the YaRN RoPE parameters as formal config fields and wiring up the corresponding CLI arguments.

HybridModel already instantiates YarnRotaryEmbedding from getattr(self.config, "yarn_*") (added in #4244), but those yarn_* attributes were never declared on TransformerConfig and had no CLI plumbing â�� they were only set dynamically in tests. As a result, launching with --position-embedding-type yarn raised AttributeError, and there was no way to configure YaRN from the command line. This PR closes that gap:

• megatron/core/transformer/transformer_config.py â�� declares the 7 YaRN fields on TransformerConfig with safe defaults (yarn_rotary_scaling_factor, yarn_original_max_position_embeddings, yarn_beta_fast, yarn_beta_slow, yarn_mscale, yarn_mscale_all_dim, yarn_correction_range_round_to_int). The default yarn_rotary_scaling_factor=1.0 is a no-op, so existing configs are unaffected.
• megatron/training/arguments.py â�� adds the matching --yarn-* arguments. Dest names match the field names, so core_transformer_config_from_args plumbs them through automatically.
• tests/unit_tests/models/test_hybrid_model.py â�� switches the YaRN test helper to set values via the constructor (the formal-field path) and adds two tests: one asserting the fields exist as dataclass fields with the expected defaults, and one asserting the config values flow into the YarnRotaryEmbedding instance.

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Issue tracking

Linked issue:

Related to #4244 (which added the YaRN wiring in HybridModel).

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