[python-package] ignore some mypy errors in _DaskLGBMModel

[jameslamb]

Contributes to #3867.

Resolves the following mypy errors.

python-package/lightgbm/dask.py:1037: error: "_DaskLGBMModel" has no attribute "_other_params"  [attr-defined]
python-package/lightgbm/dask.py:1066: error: "_DaskLGBMModel" has no attribute "get_params"  [attr-defined]
python-package/lightgbm/dask.py:1089: error: "_DaskLGBMModel" has no attribute "set_params"  [attr-defined]
python-package/lightgbm/dask.py:1095: error: "_DaskLGBMModel" has no attribute "get_params"  [attr-defined]
python-package/lightgbm/dask.py:1104: error: Item "_DaskLGBMModel" of "Union[_DaskLGBMModel, LGBMModel]" has no attribute "get_params"  [union-attr]

Those errors all come from the fact that _DaskLGBMModel is used as a mixin alongside lightgbm.sklearn.LGBMModel...

LightGBM/python-package/lightgbm/dask.py

Line 1111 in 1585ee1

class DaskLGBMClassifier(LGBMClassifier, _DaskLGBMModel):

... and references methods and attributes that it expects to come from lightgbm.sklearn.LGBMModel

LightGBM/python-package/lightgbm/dask.py

Line 1098 in 1585ee1

self._lgb_dask_copy_extra_params(self, model)

There is in theory a way to handle this situation with typing.Protocol (https://mypy.readthedocs.io/en/stable/more_types.html#mixin-classes), but I couldn't figure it out tonight. So for now, given that these methods are all covered well by the many unit tests, this PR proposes just ignoring these errors from mypy.

[jameslamb]

thanks for the reviews on all these @jmoralez ! I swear we're getting closer to having everything type-annotated and fully enabling mypy 😅

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