Support categorical data for hist. (#7695)

* Extract partitioner from hist.
* Implement categorical data support by passing the gradient index directly into the partitioner.
* Organize/update document.
* Remove code for negative hessian.

tests/python/test_updaters.py

@@ -245,3 +245,4 @@ class TestTreeMethod:
     @pytest.mark.skipif(**tm.no_pandas())
     def test_categorical(self, rows, cols, rounds, cats):
         self.run_categorical_basic(rows, cols, rounds, cats, "approx")
+        self.run_categorical_basic(rows, cols, rounds, cats, "hist")