add comments to variable.h
add comments to variable.h

HazyResearch/sampler,HazyResearch/deepdive,closedLoop/sampler,HazyResearch/sampler,sai16vicky/sampler,HazyResearch/sampler,sai16vicky/sampler,closedLoop/sampler,HazyResearch/deepdive,closedLoop/sampler,sai16vicky/sampler,HazyResearch/deepdive,closedLoop/sampler,HazyResearch/deepdive,sai16vicky/sampler