Realm backend

[chenzhuofu]

Description of changes:
Realm backend implementation (single device version).
Passed unit test and E2E test.

Related Issues:

Linked Issues:

• Issue #

Issues closed by this PR:

• Closes #

---

This change is 

[chenzhuofu]

Multi devices:

• One master proc, traverses PCG and spawns tasks on all GPUs (need to get rank info from PCG)
• Use Realm events to control dependency of tasks
• Execute a PCG within a Realm task
• Add new tests

Multi nodes:

• One master proc per node to traverse PCG
• Use barrier for dependency between nodes:

barrier = create_barrier(); // create a barrier and pass it to shard task of node 0 and node 1

// node 0:
event = launch_task(Task0);
barrier.arrive(event);

// node 1:
launch_task(Task1, barrier);

• Manage data transactions using Realm interface
• Add new tests

[chenzhuofu]

Pass E2E test!

[chenzhuofu]

I reconstruct the code and remove the redundant parts, and now it looks much simpler and clearer. The PR is ready to review now. @lockshaw @jiazhihao