Add an option for lazy loading, which will parse the data when using it. Specifically, when calling `dataset.query`, it will start to parse the query data, and `train` and `gallery` will not be parsed in advanced. This is useful when you want to run on test dataset but train dataset is large.
See examples in market1501.py
new feature that can support joint training, and find some bugs in funtion combine_all of datasets/bases.py that assume person id in dataset has been relabeld from 0 to num_class.
Another bug appears in msmt17 which trainset and testset person id both begin from 0, and we should change testset id from num_class of trainset.