Studio: revert DeepSeek OCR Image Size knob + move missing-key reset

Round 3 of the parallel-reviewer pass surfaced two issues that I had
introduced earlier in this PR's follow-ups.

- trainer.py: my prior change threaded vision_image_size into the
  DeepSeek OCR collator's image_size argument. The collator's
  (image_size, base_size, crop_mode) is a single preset
  (Tiny / Small / Base / Large / Gundam); changing image_size in
  isolation desynchronizes the per-crop pixel grid from num_queries
  downstream and produces wrong token grids on documents larger than
  the per-crop tile. The fix pins the collator back at the Gundam
  preset and logs a clear "ignored for DeepSeek OCR" notice when the
  user has selected a non-default Image Size.
- model-defaults.ts + training-config-store.ts: the round 4 fix that
  reset visionImageSize when a model YAML omitted the key also fired
  on same-model reloads (ensureModelDefaultsLoaded re-fires on page
  refresh), wiping a value the user had just selected. The reset is
  now in setSelectedModel, gated on selectedModel != previousModel,
  so true model switches still clear stale values while reloads keep
  the user's selection.

studio/backend/core/training/trainer.py

@@ -3057,22 +3057,24 @@ class UnslothTrainer:
 
                     logger.info("Configuring DeepSeek OCR data collator...\n")
                     FastVisionModel.for_training(self.model)
-                    # Honor user-selected Image Size. Keep base_size + crop_mode
-                    # at the Gundam preset (1024 / True) so dynamic cropping of
-                    # large documents still works.
-                    vision_image_size = training_args.get("vision_image_size")
-                    deepseek_image_size = (
-                        640 if vision_image_size is None else int(vision_image_size)
-                    )
-                    if vision_image_size is not None:
+                    # DeepSeek OCR's (image_size, base_size, crop_mode) tuple
+                    # is a single preset (Tiny / Small / Base / Large / Gundam).
+                    # Changing image_size in isolation desynchronizes the per-
+                    # crop pixel grid from num_queries downstream, so the
+                    # user-selected vision_image_size is intentionally ignored
+                    # here. Default to the Gundam preset, which is the
+                    # recommended training configuration. Threading the Image
+                    # Size knob through DeepSeek OCR requires patching
+                    # dynamic_preprocess first.
+                    if training_args.get("vision_image_size") is not None:
                         logger.info(
-                            f"DeepSeek OCR image resize: "
-                            f"{deepseek_image_size} (per-crop tile size)\n"
+                            "Vision image resize ignored for DeepSeek OCR "
+                            "(uses fixed Gundam preset).\n"
                         )
                     data_collator = DeepSeekOCRDataCollator(
                         tokenizer = self.tokenizer,
                         model = self.model,
-                        image_size = deepseek_image_size,
+                        image_size = 640,
                         base_size = 1024,
                         crop_mode = True,
                         train_on_responses_only = training_args.get(

studio/frontend/src/features/training/lib/model-defaults.ts

@@ -130,11 +130,10 @@ export function mapBackendModelConfigToTrainingPatch(
   const randomSeed = toNumber(training?.random_seed);
   if (randomSeed !== undefined) patch.randomSeed = randomSeed;
 
-  // Switching models must reset image size to the model default. Sibling
-  // training fields would also be reset by their respective branches above,
-  // but vision_image_size is special: model-default YAMLs in-tree currently
-  // omit the key, so without an explicit reset a stale 2048 from a previous
-  // model would silently apply to the new one.
+  // Only patch visionImageSize when the model config explicitly carries it.
+  // Resetting a stale value on model SWITCH happens in
+  // training-config-store.ts setSelectedModel, not here, so that same-model
+  // defaults reloads do not wipe a value the user just selected.
   if (Object.hasOwn(training ?? {}, "vision_image_size")) {
     const raw = training?.vision_image_size;
     if (raw == null) {
@@ -150,8 +149,6 @@ export function mapBackendModelConfigToTrainingPatch(
         patch.visionImageSize = null;
       }
     }
-  } else {
-    patch.visionImageSize = null;
   }
 
   const packing = toBoolean(training?.packing);

studio/frontend/src/features/training/stores/training-config-store.ts

@@ -502,7 +502,18 @@ export const useTrainingConfigStore = create<TrainingConfigStore>()(
         },
         setSelectedModel: (selectedModel) => {
           const previousModel = get().selectedModel;
-          set({ selectedModel, modelDefaultsError: null });
+          // True model switch resets the image size sentinel so a stale
+          // value from a previous model does not silently apply to the new
+          // one. We do this here (not in mapBackendModelConfigToTrainingPatch)
+          // so same-model defaults reloads do not wipe the user's choice.
+          const patch: { selectedModel: string | null; modelDefaultsError: null; visionImageSize?: number | null } = {
+            selectedModel,
+            modelDefaultsError: null,
+          };
+          if (selectedModel !== previousModel) {
+            patch.visionImageSize = DEFAULT_HYPERPARAMS.visionImageSize;
+          }
+          set(patch);
 
           if (!selectedModel) {
             _modelConfigController?.abort();