[enhancement]: Add Seed Variations to the Linear UI about invokeai

[psychedelicious]

The old implementation of variations used a prompt syntax. IIRC, you could have any number of seeds.

For the Linear UI graphs, I think we should mirror the exact functionality in the example graph. You get one main seed, and may blend one variation seed with it at the given alpha value. The variation seed can be manually specified or left random.

A stretch goal would allow for any number of variation seeds to be added via linear UI, but IMO that's complexity for which a use-case should be first proven. Blending between 2 seeds seems like a very reasonable feature set for the linear UI - if you want more than that, drop into workflows.

This would be a good first issue. The state and UI components are very straightforward, but it does require some graph changes. These are easy for the generation tab:

• Add the new nodes to the graph.
• Update any graph builder helper functions that depend on the noise node.

The canvas graphs are a bit more involved, because they are not using the new graph builder utils yet. The changes are a bit more complex, but not crazy.