基于MXNet的im2rec.py的debug

1.im2rec.py调试错误：multiprocessing not available, fall back to single threaded encoding imread

经过查找发现是程序中args.num_thread默认的个数是1，改为大于1的数即可

2.：mxnet.base.MXNetError: [17:31:26] src/io/input_split_base.cc:173: Check failed: files_.size() != 0U (0 vs. 0) Cannot find any files that matches the URI pattern ./data/train_depth_all_112_58532.rec