Consider changing Pretrained Dnn Image packages to not be built every day/release

The Pretrained Dnn Image packages:

- Microsoft.ML.DnnImageFeaturizer.ResNet18/
- Microsoft.ML.DnnImageFeaturizer.ResNet50/
- Microsoft.ML.DnnImageFeaturizer.AlexNet/
- Microsoft.ML.DnnImageFeaturizer.ResNet101/

Contain rather large files. The AlexNet package alone is 200MB.

However, we rebuild and republish these same large files every day in our official build:

https://dotnet.myget.org/feed/dotnet-core/package/nuget/Microsoft.ML.DnnImageFeaturizer.AlexNet

And we publish new versions of them every release:

https://www.nuget.org/packages/Microsoft.ML.DnnImageFeaturizer.AlexNet/

However, the large files in these packages haven't changed 1 time since we've enabled this feature.

We should consider splitting these packages up, or some other way of not having to republish the same large files over and over.

/cc @vaeksare @shauheen @glebuk