add --style command line option to let user choose stargan or gan-compression style (by default gan-compression style will be used).
move `dygraph.guard()` declaration into fid module for two reason: 1. the inference model didn't work in dygraph mode, so we dynamically choose whether to use dygraph mode after style is determined. 2. easier to use for end user (no need to call fluid.dygraph.guard() explicitly)