refactor: unify attention mask to single attn_mask tensor

- CudaBackend.fwd_decode passes attn_mask directly instead of kv_cache.decode_mask
- TorchNativeBackend derives pos_mask from attn_mask[:,0,0] on decode
- Drop decode_mask and page_table fields from KVCache and bind_tasks

astrai/inference/core/cache.py

@@ -203,10 +203,6 @@ class KVCache:
         seq_lens: [batch_size] — per-request total sequence lengths
         out_cache_loc: [batch, new_seq_len] or [batch, 1] — write indices
         max_len: max(seq_lens) as Python int — avoids GPU sync in decode
-        page_table: [batch, max_len] — precomputed gather indices for decode;
-            None for prefill or when not yet computed.
-        decode_mask: [batch, max_len] bool — precomputed position validity
-            mask for decode; None for prefill or single-batch decode.
     """
 
     k_buffer: Tensor
@@ -216,8 +212,6 @@ class KVCache:
     seq_lens: Tensor
     out_cache_loc: Tensor
     max_len: int = 0
-    page_table: Optional[Tensor] = None
-    decode_mask: Optional[Tensor] = None
 
 
 class PagePool:
@@ -434,21 +428,11 @@ class PagePool:
             out_cache_loc = self._req_pool.req_to_token[
                 req_pool_indices, start_pos:seq_len
             ]
-            page_table = None
-            decode_mask = None
         else:
             write_pos = seq_lens_t - 1
             out_cache_loc = self._req_pool.req_to_token[
                 req_pool_indices, write_pos
             ].unsqueeze(-1)
-            ml = max(seq_lens)
-            page_table = self._req_pool.req_to_token[req_pool_indices, :ml]
-            if len(task_ids) > 1:
-                decode_mask = (
-                    torch.arange(ml, device=device)[None, :] < seq_lens_t[:, None]
-                )
-            else:
-                decode_mask = None
 
         return KVCache(
             k_buffer=self._storage.k_buffer,
@@ -458,8 +442,6 @@ class PagePool:
             seq_lens=seq_lens_t,
             out_cache_loc=out_cache_loc,
             max_len=max(seq_lens),
-            page_table=page_table,
-            decode_mask=decode_mask,
         )
 
     # ---- internals ----