perf: 消除 RotaryEmbedding.forward 中 position_ids GPU 同步

- cos/sin 缓存预分配到 max_len，移除运行时动态扩容逻辑

- 移除未使用的 max_len_cached 属性

- 解码累计从 4.23s → 3.99s（+5.7%）

astrai/model/module.py

@@ -54,14 +54,12 @@ class RotaryEmbedding(nn.Module):
         self.dim = dim
         self.max_len = max_len
         self.base = base
-        self.max_len_cached = None
-        self._set_rotary_buffer(self.max_len, None)
+        self._set_rotary_buffer(self.max_len)
 
     def _set_rotary_buffer(self, max_len: int, device: Optional[torch.device] = None):
         cos_cached, sin_cached = get_rotary_emb(self.dim, max_len, self.base, device)
         self.register_buffer("cos_cached", cos_cached, persistent=False)
         self.register_buffer("sin_cached", sin_cached, persistent=False)
-        self.max_len_cached = max_len
 
     def forward(
         self, x: Tensor, position_ids: Optional[Tensor] = None
@@ -72,14 +70,6 @@ class RotaryEmbedding(nn.Module):
                 .unsqueeze(0)
                 .expand(x.size(0), -1)
             )
-        max_pos = position_ids.max().item()
-        if self.max_len_cached <= max_pos:
-            self._set_rotary_buffer(
-                max_pos + 1
-                if self.max_len_cached is None
-                else max(max_pos + 1, self.max_len_cached * 2),
-                x.device,
-            )
         cos = self.cos_cached[position_ids]
         sin = self.sin_cached[position_ids]
         return (cos, sin)