[MaskRCNN bug] make_data_loader() method should only return data_loaders[0] when training

[Xiao-Yamin]

Question:
The following error is reported when training MaskRCNN：

Solution：
In training\object_detection\pytorch\maskrcnn_benchmark\data\build.py, make_data_loader() method should only return data_loaders[0] when training, but now it returns data_loaders[0] and iterations_per_epoch to report this error.

[ShriyaPalsamudram]

Sorry but the maskrcnn benchmark is dropped from the training benchmarks suite so this issue cannot be addressed at this time.