[enhancement]: Suggestion to improve the Hires Fix logic for SD 1.5.

### Is there an existing issue for this?

- [X] I have searched the existing issues

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### What should this feature add?

When I generate images at 1024x1536 (2:3), the initial image is generated at 416 x 624. Here, the resolution of smaller side goes way below 512 pixels, which is not an ideal resolution for SD 1.5 finetunes since the majority are trained on images above 512 pixels and can work perfectly fine up to 768 x 768. Please modify the Hires Fix logic to generate the initial image with a minimum resolution of 512 on the smaller side. For example, if the dimensions specified by the user are 1024 x 1536 (2:3), the resolution of the initial image generated should be 512 x 768. Fallback to the current logic only when the larger side of the initial image exceeds 768 pixels, which will only happen at some aspect ratios like 16:9 or 1:2 that are quite uncommon in SD community.

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### Additional Content

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