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_shap.py

@@ -87,7 +87,6 @@ class TestSHAP(unittest.TestCase):
             r_exp = r"([0-9]+):\[f([0-9]+)<([0-9\.e-]+)\] yes=([0-9]+),no=([0-9]+).*cover=([0-9e\.]+)"
             r_exp_leaf = r"([0-9]+):leaf=([0-9\.e-]+),cover=([0-9e\.]+)"
             for tree in model.get_dump(with_stats=True):
-
                 lines = list(tree.splitlines())
                 trees.append([None for i in range(len(lines))])
                 for line in lines: