Add --cpu-offload flag for low-VRAM CUDA cards

The invisible/SynthID diffusion pass loads the whole SDXL fp16 pipeline
into VRAM via `pipeline.to("cuda")`. On an 8 GB card the weights alone
(~7 GB) leave no room for activations, so the run OOMs and there is no
in-tool way to recover short of falling back to CPU (~9 min/image).

Add an opt-in `--cpu-offload` flag (default off) that calls diffusers'
`enable_model_cpu_offload()` instead: submodules are streamed to the GPU
on demand, dropping peak VRAM to roughly the largest single submodule at
the cost of per-step transfers. CUDA-only; a no-op on cpu/mps. Threaded
through `invisible`, `all`, and `batch` to keep the knob set identical
across the three, mirroring the existing `--device`/`--pipeline` options.

Measured on a GTX 1070 Ti (8 GB): `invisible --pipeline sdxl --cpu-offload`
runs the SynthID scrub on-GPU in ~2.5 min vs ~9 min on CPU, where the
default full-VRAM path OOMs.

Test drives the placement decision with a mock pipeline (no model/GPU),
gated on torch so it runs under the `gpu` extra and skips the core CI
matrix, consistent with the model-running test policy.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/remove_ai_watermarks/cli.py

@@ -290,6 +290,16 @@ _force_option = click.option(
         "undetectable once its metadata proxy is gone)."
     ),
 )
+_cpu_offload_option = click.option(
+    "--cpu-offload/--no-cpu-offload",
+    default=False,
+    help=(
+        "Stream pipeline submodules to the GPU on demand instead of holding the whole "
+        "fp16 pipeline in VRAM (CUDA only). Lets a low-VRAM card (e.g. 8 GB) run SDXL "
+        "that would otherwise OOM, at the cost of speed. Pair with --pipeline sdxl on "
+        "the tightest cards. No effect on cpu/mps."
+    ),
+)
 
 
 _visible_backend_option = click.option(
@@ -856,6 +866,7 @@ def cmd_erase(
 @_adaptive_polish_option
 @_tile_options
 @_force_option
+@_cpu_offload_option
 @click.pass_context
 def cmd_invisible(
     ctx: click.Context,
@@ -881,6 +892,7 @@ def cmd_invisible(
     tile_size: int,
     tile_overlap: int,
     force: bool,
+    cpu_offload: bool,
 ) -> None:
     """Remove invisible AI watermarks (SynthID, StableSignature, TreeRing).
 
@@ -922,6 +934,7 @@ def cmd_invisible(
         hf_token=hf_token,
         progress_callback=progress_cb,
         controlnet_conditioning_scale=controlnet_scale,
+        cpu_offload=cpu_offload,
     )
 
     # Detect the SynthID vendor from the ORIGINAL (before processing strips C2PA) so the
@@ -1130,6 +1143,7 @@ def cmd_identify(ctx: click.Context, source: Path, no_visible: bool, as_json: bo
 @_adaptive_polish_option
 @_tile_options
 @_force_option
+@_cpu_offload_option
 @click.pass_context
 def cmd_all(
     ctx: click.Context,
@@ -1157,6 +1171,7 @@ def cmd_all(
     tile_size: int,
     tile_overlap: int,
     force: bool,
+    cpu_offload: bool,
 ) -> None:
     """Remove ALL watermarks: visible + invisible + metadata.
 
@@ -1254,6 +1269,7 @@ def cmd_all(
                 hf_token=hf_token,
                 progress_callback=progress_cb,
                 controlnet_conditioning_scale=controlnet_scale,
+                cpu_offload=cpu_offload,
             )
 
             # Detect the vendor from the pristine ORIGINAL (`source`); `tmp_path` has
@@ -1372,6 +1388,7 @@ class _BatchOptions:
     tile_size: int = 1024
     tile_overlap: int = 128
     force: bool = False
+    cpu_offload: bool = False
 
 
 def _run_batch_invisible(
@@ -1405,6 +1422,7 @@ def _run_batch_invisible(
                 pipeline=options.pipeline,
                 hf_token=options.hf_token,
                 controlnet_conditioning_scale=options.controlnet_scale,
+                cpu_offload=options.cpu_offload,
             )
         engines[options.pipeline].remove_watermark(
             img_path if mode == "invisible" else out_path,
@@ -1552,6 +1570,7 @@ def _process_batch_image(
 @_adaptive_polish_option
 @_tile_options
 @_force_option
+@_cpu_offload_option
 @click.pass_context
 def cmd_batch(
     ctx: click.Context,
@@ -1580,6 +1599,7 @@ def cmd_batch(
     tile_size: int,
     tile_overlap: int,
     force: bool,
+    cpu_offload: bool,
 ) -> None:
     """Process all images in a directory."""
     _banner()
@@ -1622,6 +1642,7 @@ def cmd_batch(
         tile_size=tile_size,
         tile_overlap=tile_overlap,
         force=force,
+        cpu_offload=cpu_offload,
     )
 
     processed = 0