feat: add SeedVR2 image and video upscaling (#2300)

docs/seedvr2.md

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+# SeedVR2 image and video super-resolution
+
+SeedVR2 upscales images and videos with `-M vid_upscale`. Support is experimental.
+
+## Weights
+
+These [ComfyUI safetensors weights](https://huggingface.co/Comfy-Org/SeedVR2)
+load directly, without conversion or a separate text encoder:
+
+| Model | `--diffusion-model` | `--vae` |
+| --- | --- | --- |
+| 3B | `seedvr2_3b_int8_convrot.safetensors` | `seedvr2_ema_vae_fp16.safetensors` |
+| 7B Sharp | `seedvr2_7b_sharp_int8_convrot.safetensors` | `seedvr2_ema_vae_fp16.safetensors` |
+
+Use the matching SeedVR2 VAE for either model. The 7B model requires more memory.
+
+## Upscale an image
+
+```sh
+sd-cli -M vid_upscale \
+  --diffusion-model models/diffusion_models/seedvr2_3b_int8_convrot.safetensors \
+  --vae models/vae/seedvr2_ema_vae_fp16.safetensors \
+  -i input.png --upscale-factor 2 -s 42 -o restored.png
+```
+
+`-i` / `--init-img` accepts a single image. Outputs can be PNG, JPEG, or WebP.
+No FPS or temporal chunk setting is needed for an image.
+
+## Upscale a video
+
+Extract the video into numbered frames, then pass the frame directory:
+
+```sh
+mkdir frames
+ffmpeg -i input.mp4 -fps_mode passthrough frames/%06d.png
+sd-cli -M vid_upscale \
+  --diffusion-model models/diffusion_models/seedvr2_3b_int8_convrot.safetensors \
+  --vae models/vae/seedvr2_ema_vae_fp16.safetensors \
+  --input-frames frames --upscale-factor 2 --fps 24 \
+  --vsr-temporal-chunk-size 1 -s 42 -o restored.avi
+```
+
+Set `--fps` to the source video's frame rate; 24 is only the example above.
+Frames are read in filename order. Use either `-i` or `--input-frames`, not both.
+
+Video outputs support AVI, WebM, and animated WebP. To save individual frames,
+use an output pattern such as `-o restored/%06d.png`.
+
+## Settings and limits
+
+- `--upscale-factor` is required and must be a positive integer. Output width
+  and height are exactly the input dimensions multiplied by this value.
+- `--vsr-temporal-chunk-size` controls how many video frames are processed
+  together. Valid values are 1, 5, 9, 13, and so on; the default is 5. A value of
+  1 processes each frame independently and uses less memory. Larger values use
+  neighboring frames but require more memory. Chunk boundaries may be visible.
+- Frame count and order are preserved, including a shorter final chunk. Audio
+  is not copied to the output.
+- If you run out of GPU memory, try `--vsr-temporal-chunk-size 1`. A five-frame
+  chunk at 1728x960 output resolution exceeded 24 GB of GPU memory in testing.
+- `--flash-attn auto` and `--conv-direct auto` are enabled by default. See
+  [backend selection](backend.md) to choose a device or weight placement.
+- SeedVR2 does not use sampler, scheduler, or step settings.
+  `--vsr-window-size` is a RelayVSR option and does not affect SeedVR2.
+
+3B has been tested on CUDA and CPU; 7B has been tested on CUDA with small inputs.
+Other backends have not been tested with this integration.
+
+Model reference: [ByteDance-Seed/SeedVR](https://github.com/ByteDance-Seed/SeedVR).