[model] feat: Add YARN support for mamba_model from MCORE

src/megatron/bridge/models/mamba/mamba_builder.py

@@ -112,10 +112,18 @@ class MambaModelConfig(ModelConfig):
     hybrid_layer_pattern: str | None = None
     seq_length: int = 8192
     # Mamba with no attention has no need for position embeddings, so none is default
-    position_embedding_type: Literal["learned_absolute", "rope", "none"] = "none"
+    position_embedding_type: Literal["learned_absolute", "rope", "yarn", "none"] = "none"
     rotary_percent: float = 1.0
     rotary_base: int = 10000
     seq_len_interpolation_factor: float | None = None
+    # YaRN parameters — only applied when position_embedding_type == "yarn"
+    yarn_rotary_scaling_factor: float = 8.0
+    yarn_original_max_position_embeddings: int | None = None
+    yarn_beta_fast: float = 32.0
+    yarn_beta_slow: float = 1.0
+    yarn_mscale: float = 1.0
+    yarn_mscale_all_dim: float = 0.0
+    yarn_correction_range_round_to_int: bool = True
     make_vocab_size_divisible_by: int = 128
     mamba_stack_spec: ModuleSpec | Callable[[], ModuleSpec] | Callable[["MambaModelConfig"], ModuleSpec] = (
         get_default_mamba_stack_spec
@@ -224,6 +232,22 @@ def build_model(
 
         pre_process = pre_process if pre_process is not None else is_pp_first_stage(pg_collection.pp)
         post_process = post_process if post_process is not None else is_pp_last_stage(pg_collection.pp)
+
+        if self._model_config.position_embedding_type == "yarn":
+            cfg = self._model_config
+            t = cfg.transformer
+            t.yarn_rotary_scaling_factor = cfg.yarn_rotary_scaling_factor
+            t.yarn_original_max_position_embeddings = (
+                cfg.yarn_original_max_position_embeddings
+                if cfg.yarn_original_max_position_embeddings is not None
+                else int(cfg.seq_length / cfg.yarn_rotary_scaling_factor)
+            )
+            t.yarn_beta_fast = cfg.yarn_beta_fast
+            t.yarn_beta_slow = cfg.yarn_beta_slow
+            t.yarn_mscale = cfg.yarn_mscale
+            t.yarn_mscale_all_dim = cfg.yarn_mscale_all_dim
+            t.yarn_correction_range_round_to_int = cfg.yarn_correction_range_round_to_int
+
         return MCoreMambaModel(
             config=self._model_config.transformer,
             mamba_stack_spec=mamba_stack_spec,

src/megatron/bridge/models/mamba/mamba_provider.py

@@ -148,11 +148,19 @@ class MambaModelProvider(TransformerConfig, ModelProviderMixin[MCoreMambaModel])
     hybrid_layer_pattern: Optional[str] = None
     seq_length: int = 8192
     # Mamba with no attention has no need for position embeddings, so none is default
-    position_embedding_type: Literal["learned_absolute", "rope", "none"] = "none"
+    position_embedding_type: Literal["learned_absolute", "rope", "yarn", "none"] = "none"
     rotary_percent: float = 1.0
     rotary_base: int = 10000
     seq_len_interpolation_factor: Optional[float] = None
     apply_rope_fusion: bool = True
+    # YaRN parameters — only applied when position_embedding_type == "yarn"
+    yarn_rotary_scaling_factor: float = 8.0
+    yarn_original_max_position_embeddings: Optional[int] = None
+    yarn_beta_fast: float = 32.0
+    yarn_beta_slow: float = 1.0
+    yarn_mscale: float = 1.0
+    yarn_mscale_all_dim: float = 0.0
+    yarn_correction_range_round_to_int: bool = True
     make_vocab_size_divisible_by: int = 128
     gated_linear_unit: bool = False
     normalization: str = "RMSNorm"
@@ -294,6 +302,9 @@ def provide(self, pre_process=None, post_process=None, vp_stage=None) -> MCoreMa
             "models due to upstream MCore MambaModel API dependency"
         )
 
+        if self.position_embedding_type == "yarn" and self.yarn_original_max_position_embeddings is None:
+            self.yarn_original_max_position_embeddings = int(self.seq_length / self.yarn_rotary_scaling_factor)
+
         assert self.vocab_size is not None, "vocab_size must be configured before calling provide()"
         if self.should_pad_vocab:
             padded_vocab_size = calculate_padded_vocab_size(

tests/unit_tests/models/mamba/test_mamba_builder.py

@@ -527,3 +527,115 @@ def test_default_parameters_forwarded(self, mock_unimodal, mock_compose):
         assert args[8] is False  # data_parallel_random_init
         assert args[9] is Float16Module  # mixed_precision_wrapper
         assert args[11] is ModelType.encoder_or_decoder  # model_type
+
+
+# =============================================================================
+# Section 4 — YaRN positional embedding support
+# =============================================================================
+
+
+class TestMambaModelConfigYarnDefaults:
+    """Tests that MambaModelConfig exposes the expected YaRN field defaults."""
+
+    def test_yarn_field_defaults(self):
+        config = _make_mamba_config()
+        assert config.yarn_rotary_scaling_factor == 8.0
+        assert config.yarn_original_max_position_embeddings is None
+        assert config.yarn_beta_fast == 32.0
+        assert config.yarn_beta_slow == 1.0
+        assert config.yarn_mscale == 1.0
+        assert config.yarn_mscale_all_dim == 0.0
+        assert config.yarn_correction_range_round_to_int is True
+
+    def test_yarn_position_embedding_type_accepted(self):
+        config = _make_mamba_config(position_embedding_type="yarn")
+        assert config.position_embedding_type == "yarn"
+
+    def test_yarn_fields_settable(self):
+        config = _make_mamba_config(
+            position_embedding_type="yarn",
+            yarn_rotary_scaling_factor=4.0,
+            yarn_original_max_position_embeddings=1024,
+            yarn_beta_fast=16.0,
+            yarn_beta_slow=0.5,
+            yarn_mscale=0.8,
+            yarn_mscale_all_dim=1.0,
+            yarn_correction_range_round_to_int=False,
+        )
+        assert config.yarn_rotary_scaling_factor == 4.0
+        assert config.yarn_original_max_position_embeddings == 1024
+        assert config.yarn_beta_fast == 16.0
+        assert config.yarn_beta_slow == 0.5
+        assert config.yarn_mscale == 0.8
+        assert config.yarn_mscale_all_dim == 1.0
+        assert config.yarn_correction_range_round_to_int is False
+
+
+class TestMambaModelBuilderBuildModelWithYarn:
+    """Tests for YaRN attribute injection in MambaModelBuilder.build_model()."""
+
+    def setup_method(self):
+        self.config = _make_mamba_config(
+            vocab_size=32000,
+            seq_length=4096,
+            position_embedding_type="yarn",
+            yarn_rotary_scaling_factor=8.0,
+        )
+        self.builder = MambaModelBuilder(self.config)
+        self.pg = Mock()
+        self.pg.pp = Mock()
+
+    @patch("megatron.bridge.models.mamba.mamba_builder.calculate_padded_vocab_size")
+    @patch("megatron.bridge.models.mamba.mamba_builder.is_pp_last_stage", return_value=True)
+    @patch("megatron.bridge.models.mamba.mamba_builder.is_pp_first_stage", return_value=True)
+    @patch("megatron.bridge.models.mamba.mamba_builder.MCoreMambaModel")
+    def test_yarn_attrs_injected_onto_transformer(self, mock_model, *_):
+        """All YaRN attrs must be set on the embedded TransformerConfig before MCoreMambaModel is called."""
+        self.builder.build_model(self.pg, pre_process=True, post_process=True)
+        t = self.config.transformer
+        assert t.yarn_rotary_scaling_factor == 8.0
+        assert t.yarn_beta_fast == 32.0
+        assert t.yarn_beta_slow == 1.0
+        assert t.yarn_mscale == 1.0
+        assert t.yarn_mscale_all_dim == 0.0
+        assert t.yarn_correction_range_round_to_int is True
+
+    @patch("megatron.bridge.models.mamba.mamba_builder.calculate_padded_vocab_size")
+    @patch("megatron.bridge.models.mamba.mamba_builder.is_pp_last_stage", return_value=True)
+    @patch("megatron.bridge.models.mamba.mamba_builder.is_pp_first_stage", return_value=True)
+    @patch("megatron.bridge.models.mamba.mamba_builder.MCoreMambaModel")
+    def test_yarn_original_max_defaulted_from_seq_length(self, mock_model, *_):
+        """None yarn_original_max_position_embeddings defaults to seq_length / scaling_factor."""
+        assert self.config.yarn_original_max_position_embeddings is None
+        self.builder.build_model(self.pg, pre_process=True, post_process=True)
+        expected = int(self.config.seq_length / self.config.yarn_rotary_scaling_factor)
+        assert self.config.transformer.yarn_original_max_position_embeddings == expected
+
+    @patch("megatron.bridge.models.mamba.mamba_builder.calculate_padded_vocab_size")
+    @patch("megatron.bridge.models.mamba.mamba_builder.is_pp_last_stage", return_value=True)
+    @patch("megatron.bridge.models.mamba.mamba_builder.is_pp_first_stage", return_value=True)
+    @patch("megatron.bridge.models.mamba.mamba_builder.MCoreMambaModel")
+    def test_yarn_original_max_explicit_value_preserved(self, mock_model, *_):
+        """An explicit yarn_original_max_position_embeddings is passed through unchanged."""
+        self.config.__dict__["yarn_original_max_position_embeddings"] = 512
+        self.builder.build_model(self.pg, pre_process=True, post_process=True)
+        assert self.config.transformer.yarn_original_max_position_embeddings == 512
+
+    @patch("megatron.bridge.models.mamba.mamba_builder.calculate_padded_vocab_size")
+    @patch("megatron.bridge.models.mamba.mamba_builder.is_pp_last_stage", return_value=True)
+    @patch("megatron.bridge.models.mamba.mamba_builder.is_pp_first_stage", return_value=True)
+    @patch("megatron.bridge.models.mamba.mamba_builder.MCoreMambaModel")
+    def test_no_yarn_injection_for_rope(self, mock_model, *_):
+        """YaRN attrs must NOT be injected when position_embedding_type is 'rope'."""
+        config = _make_mamba_config(vocab_size=32000, position_embedding_type="rope")
+        MambaModelBuilder(config).build_model(self.pg, pre_process=True, post_process=True)
+        assert not hasattr(config.transformer, "yarn_rotary_scaling_factor")
+
+    @patch("megatron.bridge.models.mamba.mamba_builder.calculate_padded_vocab_size")
+    @patch("megatron.bridge.models.mamba.mamba_builder.is_pp_last_stage", return_value=True)
+    @patch("megatron.bridge.models.mamba.mamba_builder.is_pp_first_stage", return_value=True)
+    @patch("megatron.bridge.models.mamba.mamba_builder.MCoreMambaModel")
+    def test_position_embedding_type_yarn_forwarded_to_mcore(self, mock_model, *_):
+        """position_embedding_type='yarn' must be forwarded to MCoreMambaModel."""
+        self.builder.build_model(self.pg, pre_process=True, post_process=True)
+        assert mock_model.call_args.kwargs["position_embedding_type"] == "yarn"

tests/unit_tests/models/mamba/test_mamba_provider.py

@@ -337,3 +337,121 @@ def test_finalize_uses_compatible_hybrid_layer_count(self):
 
         assert provider.num_layers == 9
         mock_finalize.assert_called_once_with(provider)
+
+
+# =============================================================================
+# YaRN positional embedding support
+# =============================================================================
+
+
+class TestMambaModelProviderYarnDefaults:
+    """Tests that MambaModelProvider exposes the expected YaRN field defaults."""
+
+    def test_yarn_field_defaults(self):
+        provider = MambaModelProvider(num_layers=2, hidden_size=128, num_attention_heads=1)
+        assert provider.yarn_rotary_scaling_factor == 8.0
+        assert provider.yarn_original_max_position_embeddings is None
+        assert provider.yarn_beta_fast == 32.0
+        assert provider.yarn_beta_slow == 1.0
+        assert provider.yarn_mscale == 1.0
+        assert provider.yarn_mscale_all_dim == 0.0
+        assert provider.yarn_correction_range_round_to_int is True
+
+    def test_yarn_position_embedding_type_accepted(self):
+        provider = MambaModelProvider(
+            num_layers=2, hidden_size=128, num_attention_heads=1, position_embedding_type="yarn"
+        )
+        assert provider.position_embedding_type == "yarn"
+
+    def test_yarn_custom_fields(self):
+        provider = MambaModelProvider(
+            num_layers=2,
+            hidden_size=128,
+            num_attention_heads=1,
+            position_embedding_type="yarn",
+            yarn_rotary_scaling_factor=4.0,
+            yarn_original_max_position_embeddings=512,
+            yarn_beta_fast=16.0,
+            yarn_beta_slow=0.5,
+            yarn_mscale=0.8,
+            yarn_mscale_all_dim=1.0,
+            yarn_correction_range_round_to_int=False,
+        )
+        assert provider.yarn_rotary_scaling_factor == 4.0
+        assert provider.yarn_original_max_position_embeddings == 512
+        assert provider.yarn_beta_fast == 16.0
+        assert provider.yarn_beta_slow == 0.5
+        assert provider.yarn_mscale == 0.8
+        assert provider.yarn_mscale_all_dim == 1.0
+        assert provider.yarn_correction_range_round_to_int is False
+
+
+class TestMambaModelProviderProvideWithYarn:
+    """Tests for YaRN handling in MambaModelProvider.provide()."""
+
+    def _make_provider(self, **kwargs):
+        defaults = dict(
+            num_layers=2,
+            hidden_size=128,
+            num_attention_heads=1,
+            vocab_size=1000,
+            tensor_model_parallel_size=1,
+            make_vocab_size_divisible_by=128,
+            position_embedding_type="yarn",
+            seq_length=4096,
+            yarn_rotary_scaling_factor=8.0,
+        )
+        defaults.update(kwargs)
+        provider = MambaModelProvider(**defaults)
+        provider._pg_collection = type("PG", (), {"pp": object()})()
+        return provider
+
+    def test_yarn_original_max_defaulted_from_seq_length(self):
+        """When yarn_original_max_position_embeddings is None, provide() fills it in."""
+        provider = self._make_provider()
+        assert provider.yarn_original_max_position_embeddings is None
+
+        with patch("megatron.bridge.models.mamba.mamba_provider.calculate_padded_vocab_size", return_value=1024):
+            with patch("megatron.bridge.models.mamba.mamba_provider.MCoreMambaModel"):
+                provider.provide(pre_process=True, post_process=True)
+
+        assert provider.yarn_original_max_position_embeddings == int(4096 / 8.0)
+
+    def test_yarn_original_max_explicit_value_preserved(self):
+        """An explicit yarn_original_max_position_embeddings is not overwritten."""
+        provider = self._make_provider(yarn_original_max_position_embeddings=256)
+
+        with patch("megatron.bridge.models.mamba.mamba_provider.calculate_padded_vocab_size", return_value=1024):
+            with patch("megatron.bridge.models.mamba.mamba_provider.MCoreMambaModel"):
+                provider.provide(pre_process=True, post_process=True)
+
+        assert provider.yarn_original_max_position_embeddings == 256
+
+    def test_no_yarn_default_injection_for_rope(self):
+        """yarn_original_max_position_embeddings should stay None when using rope."""
+        provider = MambaModelProvider(
+            num_layers=2,
+            hidden_size=128,
+            num_attention_heads=1,
+            vocab_size=1000,
+            tensor_model_parallel_size=1,
+            position_embedding_type="rope",
+        )
+        provider._pg_collection = type("PG", (), {"pp": object()})()
+
+        with patch("megatron.bridge.models.mamba.mamba_provider.calculate_padded_vocab_size", return_value=1024):
+            with patch("megatron.bridge.models.mamba.mamba_provider.MCoreMambaModel"):
+                provider.provide(pre_process=True, post_process=True)
+
+        assert provider.yarn_original_max_position_embeddings is None
+
+    def test_position_embedding_type_yarn_forwarded_to_mcore(self):
+        """position_embedding_type='yarn' must be forwarded to MCoreMambaModel."""
+        provider = self._make_provider()
+
+        with patch("megatron.bridge.models.mamba.mamba_provider.calculate_padded_vocab_size", return_value=1024):
+            with patch("megatron.bridge.models.mamba.mamba_provider.MCoreMambaModel") as mock_mamba:
+                mock_mamba.return_value = Mock()
+                provider.provide(pre_process=True, post_process=True)
+
+        assert mock_mamba.call_args.kwargs["position_embedding_type"] == "yarn"