[Bugfix] Fix Phi-3 Long RoPE scaling implementation (#5628)

Author: ShukantPal

vllm/model_executor/layers/rotary_embedding.py

@@ -507,8 +507,8 @@ def __init__(
         dtype: torch.dtype,
         short_factor: List[float],
         long_factor: List[float],
-        short_mscale: float = 1.1,
-        long_mscale: float = 1.225,
+        short_mscale: float = 1.0,
+        long_mscale: float = 1.0,
     ):
         super().__init__()
 
@@ -530,6 +530,16 @@ def __init__(
         self.short_mscale = short_mscale
         self.long_mscale = long_mscale
 
+        scale = (self.max_position_embeddings /
+                 self.original_max_position_embeddings)
+
+        if scale <= 1.0:
+            self.scaling_factor = 1.0
+        else:
+            self.scaling_factor = math.sqrt(
+                1 + math.log(scale) /
+                math.log(self.original_max_position_embeddings))
+
         short_cache = self._compute_cos_sin_cache(
             original_max_position_embeddings, short_factor, short_mscale)
         short_cache = short_cache.to(dtype)
@@ -565,8 +575,8 @@ def _compute_cos_sin_cache(
         inv_freq = self._compute_inv_freq(rescale_factors)
         t = torch.arange(max_position_embeddings, dtype=torch.float)
         freqs = torch.einsum("i,j -> ij", t, inv_freq)
-        cos = freqs.cos() * mscale
-        sin = freqs.sin() * mscale
+        cos = freqs.cos() * mscale * self.scaling_factor
+        sin = freqs.sin() * mscale * self.scaling_factor
         cache = torch.cat((cos, sin), dim=-1)
         return cache