Add constraint within treevalue structure

[HansBug]

For example, when treevalue is used in DI-treetensor, some tensors may have the same shape (all dimensions are the same) and partly-same shape (e.g. 1st dimension is the same).

Maybe a constraint should be provided, to simplify the result, such as

from treevalue import TreeValue
import torch

t = TreeValue({'a': torch.randn(2, 3), 'b': {'x': torch.randn(2, 4)}})
t.shape[0]  # should be a tree in current treevalue

# define some constraint
t.shape[0]  # should be 2 after defining