✨ Support non-instance value: class and post-hook

[Asthestarsfalll]

This PR enhance the config rule, introduce the following features:

1. Use $ prefix to mark the input is class itself instead of its instance.

   [Model.ResNet]
   $backbone = "BasicBlock"

   So the ResNet will get the <class BasicBlock> instead of instance of BasicBlock
2. Allow calling functions after an instance building, using rule:

   [Optimizer.SGD]
   @parameters = "FCN.parameters"

   So the SGD will get the outpout of method parameters() of FCN ， exempting to define confighook.
3. Above features can be combined. for example, you want to pass output of a static method, you can define cinfig:

   [A.B]
   $arg = "C.method"

   So the @ in the above optimizer example is necessary.
4. Support chained invocation like FCN.net.block.parameters

Some defects:

1. Not sure whether the @ and .method is cooperating well, more situation should be tested
2. If we want to build a module from a static method and pass its parameters to optimizer meanwhile. We cannot define config like this: $@parameters = "Model.build.parameters". So the only way to solve this situation is to use config hook or define a function witch return the output of Model.build in such a case.