Bug fixes in plot_model with nested models. (#21243)

There were several issues with `keras.utils.plot_model()` / `keras.utils.model_to_dot()` when rendering nested models with `expand_nested=True`:
- Sequential models were not expanded
- Edges going into nested models would always connect to the first box in the subgraph, which is incorrect because:
  - Functional models can have multiple inputs and the edge needs to point to the correct input
  - The destination of the edge can be further nested sub-models like Sequential sub-models
- Edges going out of nested models would always connect from the last box in the subgraph, which is incorrect because:
  - Functional models can have multiple outputs and the edge needs to come from the correct layer
  - The source of the edge can be further nested in sub-models since there is no "output" box

This adds to the integration tests. In particular, it augments the way the topology is verified:
- it provides detailed message for dangling edges
- it can inspected nested subgraphs
- it verifies there are no extra edges that the ones expected
- it can verify splits in the graph

Visual tests: https://colab.research.google.com/gist/hertschuh/fb7dfdbf4fb31139e33e750ab10aaad4/plot_model-testing.ipynb

Fixes #21119

Author: hertschuh