added support for rewriting function graphs in TF2.x models

[vlasenkoalexey]

I did some refactoring to extract common functionality from graph.py into base_graph.py and added logic to to work with functions into function_graph.py.

Work was done internally in Google, I first wanted to setup automated integration, but then decided that it is not worth it.

Tested change on TF1.15.3, 2.3 and 2.5 all tests are passing.

If you think change is too big, can move it to a separate branch.

[vlasenkoalexey]

Hi guys,

Could you please take a look at this PR. It basically adds support for rewriting functions. Technically graph_def_editor already supports TF2.x models, but it is not very useful in this case since normally TF2.x models have majority of computation graph encapsulated in functions.

This PR adds following:

• function_names property to list all functions in the graph
• get_function_graph_by_name function to return function graph. It can be manipulated similarly to a main graph. Also changing function graph arguments or adding new functions is not supported
• nodes_iterator - can iterate all nodes in the graph including nodes defined in function graphs
• breadth_first_visitor and backwards_breadth_first_visitor can now get into functions (iterate_functions arg) and get out of functions if iteration has started in the function (escape_functions arg)

I added good unit test coverage for the new functionality, and used it for multiple models. As far as I can tell everything works as expected.