replaced all dtype.np with _to_np_dtype defined in tensor.py.
after this, the only numpy usages are (1) Tensor(np.ndarray), (2) construct .numpy() output, (3) numpy random buffer
* base with bfs
* find paths
* get last
* try blocks
* Revert "try blocks"
This reverts commit 25f8e3fe85.
* this should be simpler
* full exec
* support debug
* fix lint
* add todo
* copy in_degree