* pseudo chains of OpGraph

* ConvertPseudoChainToChain

* refine pseudo_chain

* refine register coloring algorithm

* rename op_graph log file name

* remove unused code

* EnableMemSharingInVariableOp

* no mem_sharing for out_diff & model_diff in variable_op


Former-commit-id: 224bb63a0576e7da8929ab829dfcf7c709398a97