bugfix (#4493)

paddlex/inference/pipelines/formula_recognition/pipeline.py

@@ -268,7 +268,7 @@ class _FormulaRecognitionPipeline(BasePipeline):
                     layout_det_results = []
                     for _ in doc_preprocessor_images:
                         try:
-                            layout_det_res = list(external_layout_det_results)[0]
+                            layout_det_res = next(external_layout_det_results)
                         except StopIteration:
                             raise ValueError("No more layout det results")
                         layout_det_results.append(layout_det_res)