update comments in factor_graph.h
update comments in factor_graph.h

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