[TOPI] Key error in Bifrost schedule

[roastduck]

Please look at this line:

https://github.com/apache/incubator-tvm/blob/7b744b3f6b70e9323a6ea532c99e6aeb807950a2/python/tvm/topi/bifrost/conv2d.py#L321

It seems that tile_k has not been defined as a knob, so there is a key error.

Please note that this line is under an if autotvm.GLOBAL_SCOPE.in_tuning, so the error can only be triggered during auto-tuning. The error will be capture by the auto-tuner, but it won't stop the auto-tuning. It will result in a very large cost instead.

Seems relating to #5684. Can @cchung100m or @kevinthesun please have a look?