Implement tree model dump with code generator. (#4602)

* Implement tree model dump with a code generator.

* Split up generators.
* Implement graphviz generator.
* Use pattern matching.

* [Breaking] Return a Source in `to_graphviz` instead of Digraph in Python package.


Co-Authored-By: Philip Hyunsu Cho <chohyu01@cs.washington.edu>

tests/python/test_plotting.py

@@ -10,7 +10,7 @@ try:
     import matplotlib
     matplotlib.use('Agg')
     from matplotlib.axes import Axes
-    from graphviz import Digraph
+    from graphviz import Source
 except ImportError:
     pass
 
@@ -57,7 +57,7 @@ class TestPlotting(unittest.TestCase):
         assert ax.patches[3].get_facecolor() == (0, 0, 1.0, 1.0)  # blue
 
         g = xgb.to_graphviz(bst2, num_trees=0)
-        assert isinstance(g, Digraph)
+        assert isinstance(g, Source)
 
         ax = xgb.plot_tree(bst2, num_trees=0)
         assert isinstance(ax, Axes)