Fix Dask estimators serialization prior to training

[viclafargue]

Partially answers #6046

[divyegala]

Can we add a tiny test to make sure that the serialization and deserialization are both successful for un-trained models? Maybe also train the model after deserialization

[dantegd]

@viclafargue seems like the test ran into an issue in the pytest in some jobs:

=================================== FAILURES ===================================
________________________ test_serialize_before_training ________________________

client = <Client: 'tcp://127.0.0.1:45687' processes=1 threads=1, memory=251.77 GiB>

    def test_serialize_before_training(client):
        X, y = make_regression(n_samples=1000, n_features=20, random_state=0)
        X, y = da.from_array(X), da.from_array(y)
    
        model = LinearRegression(client=client)
>       pickled_model = pickle.dumps(model)

test_dask_serialization.py:90: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <cuml.dask.linear_model.linear_regression.LinearRegression object at 0x7fca696b4510>

    def __getstate__(self):
>       internal_model = self._get_internal_model().result()
E       AttributeError: 'NoneType' object has no attribute 'result'

/opt/conda/envs/test/lib/python3.11/site-packages/cuml/dask/common/base.py:60: AttributeError

[viclafargue]

@viclafargue seems like the test ran into an issue in the pytest in some jobs:

That's really strange, might possibly be missing something, but isn't there an issue with the CI?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 67.15%. Comparing base (ef7a61d) to head (39af9c2).

Files with missing lines	Patch %	Lines
python/cuml/cuml/dask/common/base.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           branch-24.12    #6065      +/-   ##
================================================
- Coverage         68.33%   67.15%   -1.18%     
================================================
  Files               193      193              
  Lines             12837    12839       +2     
================================================
- Hits               8772     8622     -150     
- Misses             4065     4217     +152     

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[divyegala]

/merge