[enhancement]: Add Seed Variations to the Linear UI

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Contact Details

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### What should this feature add?

Users have occasionally requested the ability to generate variations on a seed (something InvokeAI provided in older versions, and something that can still be done - but only in nodes workflows). Currently this can be done by using a Blend Latents node to mix two noises with two different seeds. It would be nice to have this ability available through the Linear UI, using either this method or something comparable.

In this image, the base seed of 0 is modified with a variation strength of 10% (Blend Latents Alpha of 0.1).

(Feature request created by request of @psychedelicious)

EDIT: Besides single seed variations, the feature in the old version of the linear UI used to allow combining multiple variation seed/strength pairs, which might also be nice to see return to that UI in some way

### Alternatives

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### Additional Content

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