add comments to factor_graph.cpp
add comments to factor_graph.cpp

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