early-exit: further refactoring and resnet50-imagenet

Refactor EE code and place in a separate file.
Fix resnet50-earlyexit (inputs of nn.Linear layers was wrong).

Caveats:
1. resnet50-earlyexit performance needs to be tested for performance.
2. there is still too much EE code dispersed in apputils/image_classifier.py
and compress_classifier.py