code: def _ChunkFactory(chunk_type, stream_rdr, offset):
      
      chunk_cls_map = {
          PNG_CHUNK_TYPE.IHDR: _IHDRChunk,
          PNG_CHUNK_TYPE.pHYs: _pHYsChunk,
      }
      chunk_cls = chunk_cls_map.get(chunk_type, _Chunk)
      return chunk_cls.from_offset(chunk_type, stream_rdr, offset)
docs: Return a |_Chunk| subclass instance appropriate to *chunk_type* parsed
      from *stream_rdr* at *offset*.