Txt2Img API Model Upgrade

The Text to Image API has been upgraded from Stable Diffusion v1.5 to the Pix-Art-Sigma model, a state-of-the-art DiT model that generates images in 4K resolution directly. This upgrade significantly improves image quality and alignment with text prompts compared to its predecessor, PixArt-α. Users can expect substantial enhancements in image generation using the new Txt2Img API.