Enhanced type inference and checking, called from NIRGraph.__init__

[fabio-innatera]

This is my first stab at doing type inference, in addition to type checking, on graph construction (NIRGraph.__init__).

I believe this PR will lead to some more ecosystem breakage, but in many cases I think it should improve the situation of introducing the type_check parameter recently, because many graphs should now be valid after running type inference automatically on construction.

Notes:

• Each commit should have passing tests, so we can pick-and-chose if necessary.
• This also adds nested graph type checking and inference.
• I reactivated a bunch of tests that were skipped before, now at 52 working tests with 5 skipped, up from 42 and 12 skipped.

Input and Output node handling

We automatically add Input and Output nodes if they are missing from the NIRGraph. Before, we only did this on NIRGraph.from_list, but we now do this on all graphs. This is a requirement for nested graphs, otherwise we can't validate the outputs from nested graphs against the data going in and out of it from the outer graph.

A major break is the requirement of Input and Output nodes to be present for graphs that have no non-ambiguous inputs or outputs. This is the case for certain recurrent connections, for example:

graph LR
A --> B
B --> A

Loading

For this graph, it's impossible to know which one is the input/source node and which one is the output/sink node, so the type inference will throw an error.
This graph with a recurrent connection is fine:

graph LR
input --> A
A --> B
B --> A
A --> output

Loading

[Jegp]

Excellent work! I really appreciate the thought you put into this and the number of tests you added to cover this.

I have one comment around the naming for _forward_type_inference that I'd like your opinion on. Apart from that, I couldn't find any problems. Nice!

You bring a good point that this may mess up old graphs/weirdly shaped graphs. But you neatly wrapped the inference + type checking in the constructor, so I still think it should be possible to build the graphs and ignore the type checking. I retain that the checking should be default and that any choice to ignore the typing should be a conscious choice. So, I honestly like this model.