fix: Change the replacement symbol for masked entity

Author: beanbun

graphgen/models/generator/masked_fill_in_blank_generator.py

@@ -115,7 +115,8 @@ async def generate(
         match = re.search(mask_pattern, context)
         if match:
             gth = match.group(0)
-            masked_context = mask_pattern.sub("___", context)
+            # masked_context = mask_pattern.sub("___", context)
+            masked_context = mask_pattern.sub("{ }", context)
         else:
             logger.debug(
                 "Regex Match Failed!\n"