[BUG] As there are no _tensor_to_tiled in GraphActor, referencing it will lead to AttributeError #124
Description
Describe the bug
As there are no _tensor_to_tiled in GraphActor, referencing it in
Line 773 in 9e4942c
Expected behavior
Use _get_tensor_by_key() instead. Cases are also recommended. Currently the line https://coveralls.io/builds/21024102/source?filename=mars/scheduler/graph.py#L773 is not covered.