[run_lm_finetuning] GPT2 tokenizer doesn't have a pad_token

ping @lysandrejik

examples/run_lm_finetuning.py

@@ -202,6 +202,7 @@ def mask_tokens(inputs: torch.Tensor, tokenizer: PreTrainedTokenizer, args) -> T
         tokenizer.get_special_tokens_mask(val, already_has_special_tokens=True) for val in labels.tolist()
     ]
     probability_matrix.masked_fill_(torch.tensor(special_tokens_mask, dtype=torch.bool), value=0.0)
+    if tokenizer._pad_token is not None:
         padding_mask = labels.eq(tokenizer.pad_token_id)
         probability_matrix.masked_fill_(padding_mask, value=0.0)
     masked_indices = torch.bernoulli(probability_matrix).bool()
@@ -228,6 +229,8 @@ def train(args, train_dataset, model: PreTrainedModel, tokenizer: PreTrainedToke
     args.train_batch_size = args.per_gpu_train_batch_size * max(1, args.n_gpu)
 
     def collate(examples: List[torch.Tensor]):
+        if tokenizer._pad_token is None:
+            return pad_sequence(examples, batch_first=True)
         return pad_sequence(examples, batch_first=True, padding_value=tokenizer.pad_token_id)
 
     train_sampler = RandomSampler(train_dataset) if args.local_rank == -1 else DistributedSampler(train_dataset)
@@ -421,6 +424,8 @@ def evaluate(args, model: PreTrainedModel, tokenizer: PreTrainedTokenizer, prefi
     # Note that DistributedSampler samples randomly
 
     def collate(examples: List[torch.Tensor]):
+        if tokenizer._pad_token is None:
+            return pad_sequence(examples, batch_first=True)
         return pad_sequence(examples, batch_first=True, padding_value=tokenizer.pad_token_id)
 
     eval_sampler = SequentialSampler(eval_dataset)