fix truncation_strategy for tokenizer (#1615)

Author: smallv0221

paddlenlp/transformers/tokenizer_utils.py

@@ -616,7 +616,7 @@ def __call__(self,
                 stride=stride,
                 is_split_into_words=is_split_into_words,
                 pad_to_max_seq_len=pad_to_max_seq_len,
-                truncation_strategy="longest_first",
+                truncation_strategy=truncation_strategy,
                 return_position_ids=return_position_ids,
                 return_token_type_ids=return_token_type_ids,
                 return_attention_mask=return_attention_mask,
@@ -629,7 +629,7 @@ def __call__(self,
                 text_pair=text_pair,
                 max_seq_len=max_seq_len,
                 pad_to_max_seq_len=pad_to_max_seq_len,
-                truncation_strategy="longest_first",
+                truncation_strategy=truncation_strategy,
                 return_position_ids=return_position_ids,
                 return_token_type_ids=return_token_type_ids,
                 return_attention_mask=return_attention_mask,