WebUI does not support nested search space

[Crysple]

What would you like to be added:
Show nested search space in WebUI

Why is this needed:
If NNI supports nested search space, it would be better to handle this in WebUI

Without this feature, how does current nni work：
Currently WebUI show nothing when you have a nested search space

Components that may involve changes:
WebUI

Brief description of your proposal if any:
None

[ultmaster]

Tracked in #1110.