[example] fix bucket size in example of gpt gemini (#4028)

examples/language/gpt/gemini/train_gpt_demo.py

@@ -250,7 +250,7 @@ def main():
         plugin = None
         if args.distplan.startswith("CAI_ZeRO"):
             plugin = LowLevelZeroPlugin(stage=zero_stage,
-                                        reduce_bucket_size_in_m=12 * 1024 * 1024,
+                                        reduce_bucket_size_in_m=12,
                                         overlap_communication=True,
                                         verbose=True)
         elif args.distplan == "CAI_Gemini":