[python] Use double type for init_score array when set by predictor

[StrikerRUS]

I'm not sure but looks like init_score is supposed to be only double type.

• LGBM_BoosterPredictFor* functions return predictions in double type
• https://lightgbm.readthedocs.io/en/latest/C-API.html#c.LGBM_DatasetSetField

  init_score only works for C_API_DTYPE_FLOAT64

• LightGBM/python-package/lightgbm/basic.py

  Line 446 in cfe8eb1

  "init_score": C_API_DTYPE_FLOAT64,

• LightGBM/python-package/lightgbm/basic.py

  Lines 1919 to 1920 in cfe8eb1

  	elif field_name == 'init_score':
  	dtype = np.float64

[jameslamb]

makes sense to me

[github-actions]

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