Three kinds of distribution policy

[yanyh15]

1. one time decision making of loop distribution: BLOCK and MODEL_AUTO
2. two times decision making of loop distribution: MODEL_PROFILE_AUTO, SCHED_PROFILE_AUTO.
   In these two policies, distribute a small amount of chunks first, profile it and then distribute the rest
3. Multiple times decision making of loop distribution: SCHED_DYNAMIC, SCHED_GUIDED

Currently we use a single map object to keep track of data that are distributed in those policies that need more than one decision making. One alternative is to create and use a new map each time we need to distribute data or loop.