Enforce 4-way padding in topi/nn/conv2d. PR-4644 is already merged

There are couple TODOs related to PR-4644 (Merged) - TODO enforce 4-way padding in topi/nn/conv2d after #4644 merged

Looks like we can address these two TODOs now since PR-4644 was merged on Feb 24, 2020

@comaniac @haichen @xyzhou

@xyzhou as you are the TODO author, could you take a look?

Good catch! Currently I do not have bandwidth to work on this TODO until next week, unfortunately. It would be great if you could help fix them if that’s urgent to you