set tgt_lang of MBart Tokenizer for summarization (#10205)

examples/seq2seq/run_seq2seq.py

@@ -386,7 +386,7 @@ def main():
 
     # For translation we set the codes of our source and target languages (only useful for mBART, the others will
     # ignore those attributes).
-    if data_args.task.startswith("translation"):
+    if data_args.task.startswith("translation") or isinstance(tokenizer, (MBartTokenizer, MBartTokenizerFast)):
         if data_args.source_lang is not None:
             tokenizer.src_lang = data_args.source_lang
         if data_args.target_lang is not None: