Add simple HPO search space validation

[liuzhe-lz]

There are multiple issues caused by invalid user search space.
Add a simple validation to make these errors more obvious.

I'm not confident about current implementation, so it only prints an error log and does not interrupt tuner.

[acured]

If we never use the return value, change "validate_search_space" as void should be ok?

[liuzhe-lz]

As a public API, the caller might want validation result to decide following action.
For example, I think it's better to integrate validation into tuner API, and then perform early validation in nnictl. In this case, nnictl should halt launching when validation failed, but it won't want to raise the exception to end user.