docs: 📝 document dilate_gpu (GPU dilation on the cluster)

Explains dilate_gpu: true, its cupy + GPU-allocation requirements, and
that it's independent of whatever method itself runs on.

Author: lguerard

docs/guide/custom_segmentation.md

@@ -74,6 +74,15 @@ tiles are merged, so `overlap` must still cover the dilation amount. On the
 cluster, set `dilate: N` in the config instead — see [Configure the
 run](snakemake.md#3-configure-the-run).
 
+By default the dilation itself runs on CPU (scipy), independent of whatever
+backend `fn` used — pass `use_gpu=True` to dilate via cupy instead:
+
+```python
+fn = dilate_labels(fn, iterations=2, use_gpu=True)  # needs cupy installed
+```
+
+On the cluster, this is the `dilate_gpu: true` config key.
+
 ## Real example: StarDist 3-D, with model caching
 
 Heavy models must be loaded **once**, not per tile. On SLURM each tile is its

docs/guide/snakemake.md

@@ -65,6 +65,7 @@ empty_threshold: null          # null → Otsu
 method: "cellpose"             # "cellpose" (GPU), "threshold" (no GPU), "custom"
 label_name: "cellpose"         # name under image.zarr/labels/
 dilate: 0                      # optional: pixels to grow labels by, any method
+dilate_gpu: false               # dilate via cupy instead of scipy (needs a GPU)
 cellpose:
   model: "cyto3"
   diameter: 30
@@ -80,8 +81,15 @@ sequential_labels: true        # renumber labels to a contiguous 1..N
 
 !!! tip "Growing labels after segmentation"
     `dilate: N` grows every label by `N` pixels once segmentation finishes,
-    regardless of `method`. `0` (default) disables it. See [Growing labels
-    afterwards](custom_segmentation.md#growing-labels-afterwards-dilation)
+    regardless of `method`. `0` (default) disables it. Runs on CPU (scipy)
+    by default; set `dilate_gpu: true` to dilate via cupy instead — that
+    needs `cupy` installed in the segment job's environment (matching your
+    CUDA version, e.g. `pip install cupy-cuda12x`) and a GPU allocated for
+    that job (`set-resources: segment:` in `profile/slurm/config.yaml`,
+    same as for a GPU `method`). It's independent of whatever `method`
+    itself runs on — you can dilate on GPU even with `method: "threshold"`
+    (CPU), or on CPU even with `method: "cellpose"` (GPU). See [Growing
+    labels afterwards](custom_segmentation.md#growing-labels-afterwards-dilation)
     for how it works and the equivalent direct-API call.
 
 !!! tip "Tile size vs runtime"