message: Allow rematerialization method to be overridden.
  This change allows users to more easily subclass HloRematerialization and define new rematerialization methods.
diff: @@ -79,7 +79,7 @@ class HloRematerialization : public HloModulePass {
  // order in which the computation's instructions will be emitted in the
  // backend. Rematerialized instructions will be added to the HLO computation
  // and inserted into 'order'.
  - StatusOr<bool> RematerializeComputation(HloComputation* computation,
  + virtual StatusOr<bool> RematerializeComputation(HloComputation* computation,
  HloSchedule* schedule,
  int64 memory_limit_bytes);
  