code: def _cpu_buffer(self, idx):
      if self._cpu_buffers[idx] is None:
          numpy_data = np.zeros(
              (self.buffer_size, self.emb_dim), dtype=np.float32
          )
          self._cpu_buffers[idx] = paddle.to_tensor(
              numpy_data, place=paddle.CUDAPinnedPlace()
          )
      return self._cpu_buffers[idx]
  
imports: import numpy
  import paddle
  from paddle.framework import core
  from paddle.device import cuda
  from pgl.utils.logger import log
label_desc: Use image node_id 4 for calling the StreamPool obj's underlying member method code with example usage: obj._cpu_buffer(idx) and returns: self