text: Title: Support of num_workers (multiprocessing) in map for IterableDataset
  Body: ### Feature request
  
  Currently, IterableDataset doesn't support setting num_worker in .map(), which results in slow processing here. Could we add support for it? As .map() can be run in the batch fashion (e.g., batch_size is default to 1000 in datasets), it seems to be doable for IterableDataset as the regular Dataset.
  
  ### Motivation
  
  Improving data processing efficiency
  
  ### Your contribution
  
  Testing