[CoreML] Solve CoreML frontend issue of image scaler and padding

[FrozenGene]

We have issue of image scaler and padding in CoreML frontend, so our mobilenet model has incorrect classify result. see issue #3725

Also update the tutorial of CoreML frontend, mobilenet model input image format is BGR.

[tqchen]

Thanks @FrozenGene

[FrozenGene]

I find the codebase of master branch's CI is red (http://ci.tvm.ai:8080/blue/organizations/jenkins/tvm/detail/master/1459/pipeline) after merged, however, CI is green as seen when PR is made and I can not reproduce the issue. @tqchen Do we need to restart the CI?

[FrozenGene]

After tried it about ~100 times, I reproduce it. It is one occasionally problem and seems only happened on CUDA (with Cython3 enabled). When we turn on opt_level=3, we call alter_op_layout pass and when we insert one pad with all zeros (i.e. pad(0,0,0,0), we will meet layout problem. However, it shouldn't be. I think I could do one quickfix in coreml frontend. (i.e. When we meet all pad values is zero, we avoid insert pad). I will make one pr tonight. @tqchen

I will try to spare time to construct one reproduce issue and try to find the root cause then fix it.