perf: 消除 PagedCache.write 中的 position_ids GPU 同步，解码提速 15%

- CacheView.write 用 total_len - k.size(1) 推导 start_pos，替代 position_ids[0,0].item()

- 移除 GQA/MLA/DecoderBlock 中不再使用的 position_ids 参数

- PagedCache.write 参数 position_ids:Tensor → start_pos:int

astrai/model/transformer.py

@@ -132,7 +132,7 @@ class Transformer(AutoModel):
         attn_mask = process_attention_mask(x, position_ids, input_mask, is_causal=True)
 
         for layer in self.layers:
-            x = layer(x, rotary_emb, attn_mask, paged_cache, position_ids)
+            x = layer(x, rotary_emb, attn_mask, paged_cache)
 
         hidden_states = self.norm(x)
         logits = self.lm_head(hidden_states)