code: def _push_stream(self, idx: int) -> Stream:
      if self._push_streams[idx] is None:
          assert str(self._device)[:4] == "cuda"
          self._push_streams[idx] = torch.cuda.Stream(self._device)
      return self._push_streams[idx]
  
imports: from typing import Optional, Callable
  import torch
  from torch import Tensor
  from torch.cuda import Stream
  from dig_ext.sync import synchronize, read_async, write_async
label_desc: Use image node_id 4 for calling the AsyncIOPool obj's underlying member method code with example usage: obj._push_stream(idx) and returns: self