Better separation of configurations and values

[bpiwowar]

In some cases, it is better to distinguish configurations from instances - e.g. when using deriving from torch.nn.Module. In those cases, it would be better to have an explicit split between both.

class A(Config):
    x: Param[int]

@A.instance()
class InstanceA(A):
   def __post_init__(self): 
       pass

class B(A):
    y: Param[int]

@B.instance()
class BInstance(B, A.XPMValue):
    pass

[bpiwowar]

Fixed in 90ffc04