class_docstr: Dataloader which wraps another dataloader, and moves the data to GPU
  in async manner so as to overlap the cost of copying data from
  cpu to gpu with the previous model iteration.
code: def __len__(self) -> int:
      return len(self.dataloader)
  
file_path: vissl/vissl/data/dataloader_sync_gpu_wrapper.py
imports: from typing import Any, Iterable, Iterator
  import torch
  from classy_vision.dataset.dataloader_wrapper import DataloaderWrapper
  from classy_vision.generic.util import recursive_copy_to_gpu
label_desc: Use image node_id 4 for calling the DataloaderSyncGPUWrapper obj's underlying member method code with example usage: obj.__len__() and returns: len