Transformers 4.21.0: Can't load XLMRoberta checkpoints
### System Info

Transformers 4.21.0

### Who can help?

@LysandreJik @sgugger 

### Information

- [ ] The official example scripts
- [X] My own modified scripts

### Tasks

- [X] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [ ] My own task or dataset (give details below)

### Reproduction

1. cont_pre_model = XLMRobertaForMaskedLM.from_pretrained('xlm-roberta-base')
2.     cont_pre_training_args = TrainingArguments(
        output_dir=temp_dir,
        num_train_epochs=40,
        per_device_train_batch_size=4,
        save_steps=5000,
        logging_steps=50,
        save_total_limit=3,
        prediction_loss_only=True,
        evaluation_strategy='no',
        learning_rate=2e-5,
        warmup_steps=cont_pre_warmup_steps,
        dataloader_num_workers=0,
        disable_tqdm=False,
        gradient_accumulation_steps=8,
        fp16=True
    )
3.     cont_pre_trainer = Trainer(
        model=cont_pre_model,
        args=cont_pre_training_args,
        train_dataset=cont_pre_dataset,
        data_collator=cont_pre_collator
    )
Then start to train, cont_pre_trainer.train(), save some checkpoints
4. Corrupt it, try to continue from checkpoint
cont_pre_trainer.train(resume_from_checkpoint=True)

### Expected behavior

Describe:
I used XLMRobertaForMaskedLM.from_pretrained('xlm-roberta-base') to continue pretraining, the training process was too long, so I save checkpoints regularly. I did this in Google Colab.
Several days ago, I can't load any saved checkpoints by using "cont_pre_trainer.train(resume_from_checkpoint=True)", there is always such an error:
RuntimeError: Error(s) in loading state_dict for XLMRobertaForMaskedLM:
Missing key(s) in state_dict: "lm_head.decoder.weight", "lm_head.decoder.bias".

The reason:
XLMRobertaForMaskedLM doesn't have "lm_head.decoder.weight", "lm_head.decoder.bias". And state_dict of a PyTorch module is an OrderedDict and it complains about missing keys. Maybe you should use such a command somewhere: load_state_dict(state_dict, strict=False)

How to solve it by myself:
I rollbacked transformers to version 4.20.1 and it worked then.

Problem Conclusion:
Transformers version 4.21.0 can't load checkpoints that trained on both version 4.20.1 and version 4.21.0.
(Transformers version 4.20.1 works normally, I use it to process checkpoints trained on version 4.20.1 or version 4.21.0)
cc @sgugger <|||||>We don't support resuming training with a different version of Transformers that initiated it, as it would require just freezing the whole `Trainer` forever: any bug fix or feature added in it won't work with a resumed checkpoint.<|||||>I am facing the same error with Transformers version 4.21.0 - model trained on same transformers version and loading the best model after training gives this error. I am using `xlm-roberta-base` with `AutoModelForMaskedLM`

`RuntimeError: Error(s) in loading state_dict for XLMRobertaForMaskedLM:
	Missing key(s) in state_dict: "lm_head.decoder.weight", "lm_head.decoder.bias".`
<|||||>Thanks for reporting @harshit-sethi09, with the initial report I thought this was a change in the XLM-RoBERTa model that was causing problems across versions but the whole reload is broken in 4.21.0 because of the changes in #18221 .

The PR mentioned above should fix it and we will soon make a patch release with it.