Tweaks to VisualizeTree to make it tractable to render complex training graphs (#2864)

* Some hacks for graph visualization

* More betterer

* Remove debug prints

* Cheap temporary hack to deal with zombie users issue

* Less hacky fix for the zombie users issue

* Change to use get_name instead of get_friendly_name; misc. cleanup of dot output

* Allow '.dot' as VisualizeTree output format, and make it default

* Cleanup, and some documentation of this jump distance nonsense

* Comment edits

* Another comment edit

* Fix typo

* Remove reverse_topological_sort function (it's not needed)

src/ngraph/pass/manager.cpp

@@ -160,19 +160,18 @@ void pass::Manager::run_passes(shared_ptr<Function> func, bool transitive)
             std::string index_str = std::to_string(index);
             index_str = std::string(num_digits_in_pass_index - index_str.length(), '0') + index_str;
             auto base_filename = f_array.at(0)->get_name() + std::string("_") + index_str +
-                                 std::string("_") + m_pass_names.at(index) + std::string(".");
+                                 std::string("_") + m_pass_names.at(index);
 
             if (m_visualize)
             {
-                pass::VisualizeTree vt(base_filename + pass::VisualizeTree::get_file_ext());
+                pass::VisualizeTree vt(base_filename);
                 vt.set_ops_to_details(get_state().get_visualize_tree_ops_map());
                 vt.run_on_module(f_array);
             }
 
             if (m_serialize)
             {
-                // no "." in the extension
-                pass::Serialization st(base_filename + "json");
+                pass::Serialization st(base_filename + ".json");
                 st.run_on_module(f_array);
             }
         }