Add BOHB Advisor

[PurityFan]

• Add BOHB Advisor

1. Add new advisor with new algorithm BOHB, reference paper: https://arxiv.org/abs/1807.01774
2. Add MNIST example of BOHB usage
3. Add related docs and links

[leelaylay]

@PurityFan I have one simple suggestion that you can change the _KEY = 'TRIAL_BUDGET' to _KEY = 'STEPS'(Hyperband uses this name).

[PurityFan]

@PurityFan I have one simple suggestion that you can change the _KEY = 'TRIAL_BUDGET' to _KEY = 'STEPS'(Hyperband uses this name).

We decided to change the “STEPS” uniform to “TRIAL_BUDGET” at the previous meeting.

[leelaylay]

I can run it successfully. Hope for approval.

[QuanluZhang]

doc can be further improved

[xuehui1991]

I suggest that more people to review this big PR.

[leckie-chn]

would be better if we could leverage existing config parsing code.

[PurityFan]

BOHB needs to use the functions in ConfigSpace, this function is to convert the format represented by nni into a format that can be recognized in the ConfigSpace package. Not clear of how to leverage existing config parsing code?