GPTuner does not support string choices

[mys007]

Short summary about the issue/question:

GPTuner raises an exception when search space contains a choice over strings:

{
    ....
    "model_config": {"_type": "choice", "_value": ["A", "B"]}
}

......
  File "/home/xxx/miniconda3/envs/spur/lib/python3.7/site-packages/nni/tuner.py", line 52, in generate_multiple_parameters
    res = self.generate_parameters(parameter_id, **kwargs)
  File "/home/xxx/miniconda3/envs/spur/lib/python3.7/site-packages/nni/gp_tuner/gp_tuner.py", line 101, in generate_parameters
    results = self._space.random_sample()
  File "/home/xxx/miniconda3/envs/spur/lib/python3.7/site-packages/nni/gp_tuner/target_space.py", line 180, in random_sample
    _bound['_value'], self.random_state)
ValueError: could not convert string to float: 'A'

This is caused by https://github.com/microsoft/nni/blob/master/src/sdk/pynni/nni/gp_tuner/target_space.py#L176, which enforces float types. I believe replacing the line with params = []*self.dim could solve the issue?

nni Environment:

• nni version: 1.0
• nni mode(local|pai|remote): local
• OS: Ubuntu
• python version: 3.7
• is conda or virtualenv used?: conda
• is running in docker?: no

[scarlett2018]

@mys007 - Thanks for raising the problem, this is a known limitation, our current GPTuner reference implementation only support choice for integer and float. @suiguoxin will update the document to let users know about this limitation.

[rabbit008]

#1551 add doc for GP-Tuner: support only numerical values