[backport] Use maximum category in sketch. (#7853) (#7866)

tests/python-gpu/test_gpu_updaters.py

@@ -61,6 +61,9 @@ class TestGPUUpdaters:
     def test_categorical(self, rows, cols, rounds, cats):
         self.cputest.run_categorical_basic(rows, cols, rounds, cats, "gpu_hist")
 
+    def test_max_cat(self) -> None:
+        self.cputest.run_max_cat("gpu_hist")
+
     def test_categorical_32_cat(self):
         '''32 hits the bound of integer bitset, so special test'''
         rows = 1000