[dask] dask cudf inplace prediction. (#5512)

* Add inplace prediction for dask-cudf.

* Remove Dockerfile.release, since it's not used anywhere

* Use Conda exclusively in CUDF and GPU containers

* Improve cupy memory copying.

* Add skip marks to tests.

* Add mgpu-cudf category on the CI to run all distributed tests.

Co-authored-by: Hyunsu Cho <chohyu01@cs.washington.edu>

tests/python-gpu/test_gpu_prediction.py

@@ -62,6 +62,7 @@ class TestGPUPredict(unittest.TestCase):
 
     # Test case for a bug where multiple batch predictions made on a
     # test set produce incorrect results
+    @pytest.mark.skipif(**tm.no_sklearn())
     def test_multi_predict(self):
         from sklearn.datasets import make_regression
         from sklearn.model_selection import train_test_split
@@ -89,6 +90,7 @@ class TestGPUPredict(unittest.TestCase):
         assert np.allclose(predict0, predict1)
         assert np.allclose(predict0, cpu_predict)
 
+    @pytest.mark.skipif(**tm.no_sklearn())
     def test_sklearn(self):
         m, n = 15000, 14
         tr_size = 2500