class_docstr: Memory Efficient RowReader 
      
code: def __getitem__(self, key):
      if key not in self.loaded_nfeat:
          self.loaded_nfeat[key] = read_rows(
              self.nfeat[key], self.index
          )
      return self.loaded_nfeat[key]
  
file_path: PGL/legacy/pgl/utils/op.py
imports: import numpy
  import paddle.fluid
  from pgl.utils import paddle_helper
label_desc: Use image node_id 2 for calling the RowReader obj's underlying member method code with example usage: obj.__getitem__(key) and returns: self