Replace RuntimeError in _lookup_task with deferred error. (#8421)
Replace RuntimeError in _lookup_task with deferred error. (#8421)

* Replace RuntimeError in _lookup_task with deferred error.

This allows unknown tasks to be created (e.g., when parsing
autotvm log files) but not invoked.

* Format.

* Update python/tvm/autotvm/task/task.py

Co-authored-by: Cody Yu <a293e2a7a87d2c31d7aadfa45210e3a2063d0cc8@gmail.com>

Co-authored-by: Matt Welsh <b35459fcbfd785b102e4d0328c70c35968267348@mdw.la>
Co-authored-by: Cody Yu <a293e2a7a87d2c31d7aadfa45210e3a2063d0cc8@gmail.com>
Laurawly/tvm-1,Laurawly/tvm-1,dmlc/tvm,dmlc/tvm,Laurawly/tvm-1,Laurawly/tvm-1,dmlc/tvm,dmlc/tvm,dmlc/tvm,Laurawly/tvm-1,dmlc/tvm,dmlc/tvm,dmlc/tvm,dmlc/tvm,Laurawly/tvm-1,Laurawly/tvm-1,Laurawly/tvm-1,Laurawly/tvm-1,Laurawly/tvm-1