code: def _cpu_buffer(self, idx: int) -> Tensor:
      if self._cpu_buffers[idx] is None:
          self._cpu_buffers[idx] = torch.empty(
              self.buffer_size, self.embedding_dim, pin_memory=True
          )
      return self._cpu_buffers[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 5 for calling the AsyncIOPool obj's underlying member method code with example usage: obj._cpu_buffer(idx) and returns: self