Created by: heavengate

fix train finish pyreader reset error train_pyreader.reset() will call reader.reset(), MappedDataset not implement reset() currently, which may cause following error:

Traceback (most recent call last):
  File "/ssd3/xiege/models/PaddleCV/PaddleDetection/ppdet/data/transform/transformer.py", line 44, in _proxy_method
    return func(*args, **kwargs)
  File "/ssd3/xiege/models/PaddleCV/PaddleDetection/ppdet/data/dataset.py", line 48, in reset
    (self.__class__.__name__))
NotImplementedError: MappedDataset.reset not available