fix acc topk's handling of the case when dim=0, fix tests as well (#64727)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64727
the acc ops convertor for topk has a subtle bug (i found this while trying to introduce max/min)
the code does not differentiate between dim == None and dim ==0, but these are both different computations
Reviewed By: jfix71, 842974287
Differential Revision: D30833621
fbshipit-source-id: 6cd84e6ca4e95bb1a6d6465e61830b76808a9c78
