predict_single_npy_array: Copy warning from inference/readme.md

nnunetv2/inference/predict_from_raw_data.py

@@ -422,6 +422,8 @@ def predict_single_npy_array(self, input_image: np.ndarray, image_properties: di
                                  output_file_truncated: str = None,
                                  save_or_return_probabilities: bool = False):
         """
+        WARNING: SLOW. ONLY USE THIS IF YOU CANNOT GIVE NNUNET MULTIPLE IMAGES AT ONCE FOR SOME REASON.
+
         input_image must use SimpleITK axis ordering!
             (NB: if array comes from a nibabel-loaded image, you must swap the
              axes of both the array *and* the spacing from [x,y,z] to [z,y,x]!)