Explicitly validate node layers for mlp builds

[danbraunai-apollo]

tests.test_build_graph.test_mnist_build_graph_invalid_node_layers currently passes (tests that out of order node layers won't build correctly), but the issue is that there is no explicit test for this before building the graph. The code will actually run until it gets to calculating the edges.

This should be fixed if we implement a SequentialMLP which would validate node layers on initialisation.