Test tensor execution in distributed mode

[hekaisheng]

Is your feature request related to a problem? Please describe.
Currently, our expressions test cases all run by thread-based executor, however the behavior changes when running in cluster, only a few cases in test_api and test_main works for end to end.

Several issues(#19 #64 #8 ) are related to this, maybe we need some tests to cover them in the future.

Describe the solution you'd like
At least, we can add cases in testGraphActor to test GraphActor with different input tensors.

[qinxuye]

If most failures are due to the serialization, maybe we need to enhance the tests in tensor/expressions.

The e2e tests are quite heavy, I think we only need to test on some special and corner cases.

[wjsi]

We may test under a mimic setting for the distributed cases in an actor-free environment. That is,

1. Serialize / deserialize the tensor graph;
2. Tile with the same code as GraphActor used;
3. Extract operands by get_executable_operand_dag;
4. Execute operands one by one.

[qinxuye]

This issue would be replaced by #267 , close first.