Reorganize and simplify AdaptDL-PyTorch tutorial

[aurickq]

Currently the AdaptDL-PyTorch document is a sequence of 6-7 steps, it would be more readable to re-organize to 3 sections. First section on "simple" usage of only AdaptiveDataParallel, AdaptiveDataLoader, and remaining_epochs_until. Second section on adaptive batch size, and third section on "advanced" usage of Accumulator. We only need three example training scripts, one for each of the sections, and all three are completely runnable.