[ci] [python-package] fix mypy errors about Dataset._set_init_score_by_predictor()

[jameslamb]

Contributes to #3867.

Fixes the following errors from mypy.

python-package/lightgbm/basic.py:1750: error: Item "List[Any]" of "Union[ndarray[Any, Any], Any, List[Any]]" has no attribute "ravel"  [union-attr]
python-package/lightgbm/basic.py:1768: error: Item "List[float]" of "Union[List[float], Any, List[List[float]], ndarray[Any, Any]]" has no attribute "shape"  [union-attr]
python-package/lightgbm/basic.py:1768: error: Item "List[List[float]]" of "Union[List[float], Any, List[List[float]], ndarray[Any, Any]]" has no attribute "shape"  [union-attr]

[github-actions]

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