Separate schedulers and searchers as a two separate inputs in ImageClassification task

[gradientsky]

AutoGluon separated scheduler and searcher as two different inputs into HPO. However, in the code [1] only search_strategy is available. Currently we have to do this workaround: [2][3].

[1] https://github.com/dmlc/gluon-cv/blob/master/gluoncv/auto/tasks/image_classification.py#L234-L259
[2] https://github.com/awslabs/autogluon/blob/45a8f9166d3a791e6c65b723748ea92892b3b679/vision/src/autogluon/vision/predictor/predictor.py#L282-L284
[3] https://github.com/awslabs/autogluon/blob/45a8f9166d3a791e6c65b723748ea92892b3b679/vision/src/autogluon/vision/predictor/predictor.py#L317-L319