transform in data api

[mli]

@zhreshold @zhanghang1989

i remember gluon has changed the api to transform_first instead of passing to transform?

also target_transform is confusing. shouldn't transform can also transform the label?

[zhreshold]

Detection is not going to use transform when instantiate datasets, do you plan to strip it in segmentation? @zhanghang1989

[zhanghang1989]

The target transform is removed, and the data format is handled inside. #63