[Bugfix] Fix dynamic rotary embedding (#20343)

Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>

Author: DarkLight1337

vllm/model_executor/layers/rotary_embedding.py

@@ -1963,16 +1963,19 @@ def get_rope(
                                                    scaling_factor, dtype,
                                                    mixed_b)
         elif scaling_type == "dynamic":
-            scaling_factor = rope_scaling["factor"]
-            scaling_alpha = rope_scaling["alpha"]
-            if scaling_alpha:
+            if "alpha" in rope_scaling:
+                scaling_alpha = rope_scaling["alpha"]
                 rotary_emb = DynamicNTKAlphaRotaryEmbedding(
                     head_size, rotary_dim, max_position, base, is_neox_style,
                     scaling_alpha, dtype)
-            else:
+            elif "factor" in rope_scaling:
+                scaling_factor = rope_scaling["factor"]
                 rotary_emb = DynamicNTKScalingRotaryEmbedding(
                     head_size, rotary_dim, max_position, base, is_neox_style,
                     scaling_factor, dtype)
+            else:
+                raise ValueError("Dynamic rope scaling must contain either "
+                                 "'alpha' or 'factor' field")
         elif scaling_type == "yarn":
             scaling_factor = rope_scaling["factor"]
             original_max_position = rope_scaling[