Explore the possibility of adding Checkpoint Loader Mixins

[Alicegaz]

There are cases where multiple nn.Module components are involved in a training process, some of which are never trained but used in an eval mode, in a preprocessing, for instance (detection, neural augmenters, batch preprocessor). In these cases, we would have to override constructors for those components, but why not abstract all the checkpoint loading logic inside class Mixins, and the instantiate components that need checkpoint loading logic from the corresponding Mixin. In this case, all the checkpoint loading parameters for the mixin are placed as module parameters in config.