MAINT make it explicit that additive_chi2_kernel does not accept sparse matrix

Author: glemaitre

sklearn/metrics/pairwise.py

@@ -1555,7 +1555,7 @@ def additive_chi2_kernel(X, Y=None):
       International Journal of Computer Vision 2007
       https://hal.archives-ouvertes.fr/hal-00171412/document
     """
-    X, Y = check_pairwise_arrays(X, Y)
+    X, Y = check_pairwise_arrays(X, Y, accept_sparse=False)
     if (X < 0).any():
         raise ValueError("X contains negative values.")
     if Y is not X and (Y < 0).any():