Fix FFDetrDetector crash on single-item final batch (#31)

Author: Teekyy

commonforms/inference.py

@@ -74,9 +74,10 @@ def extract_widgets(
         results = []
         for b in batch([p.image for p in pages], n=batch_size):
             predictions = self.model.predict(b, threshold=confidence)
-            if len(pages) == 1 or batch_size == 1:
-                predictions = [predictions]
-            results.extend(predictions)
+            if isinstance(predictions, list):
+                results.extend(predictions)
+            else:
+                results.append(predictions)
 
         widgets = {}
 
@@ -264,9 +265,14 @@ def prepare_form(
     except pypdfium2._helpers.misc.PdfiumError:
         raise EncryptedPdfError
 
-    results = detector.extract_widgets(
-        pages, confidence=confidence, image_size=image_size
-    )
+    if isinstance(detector, FFDetrDetector):
+        results = detector.extract_widgets(
+            pages, confidence=confidence, image_size=image_size, batch_size=batch_size
+        )
+    else:
+        results = detector.extract_widgets(
+            pages, confidence=confidence, image_size=image_size
+        )
 
     writer = PyPdfFormCreator(input_path)
     if not keep_existing_fields: