ProphetNet with AttributeError: module 'torch.utils' has no attribute 'checkpoint'
## Environment info

- `transformers` version: 4.5.0
- Platform: Linux-5.4.0-70-generic-x86_64-with-debian-buster-sid
- Python version: 3.7.6
- PyTorch version (GPU?): 1.7.1 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: Yes
- Using distributed or parallel set-up in script?: No

### Who can help
@patrickvonplaten

## Information

Model I am using ProphetNet:

The problem arises when using:
my own modified scripts (simplified): 
```python
self.model = ProphetNetForConditionalGeneration.from_pretrained(self.pretrained_model_path, config=self.config)
outputs = self.model(
            input_ids,
            attention_mask=input_att,
            decoder_input_ids=decoder_input_ids,
            decoder_attention_mask=decoder_input_att,
            use_cache=False
        )
```
And rised: 
```
File "/home/ruc/tty/TextBox/textbox/model/Seq2Seq/prophetnet.py", line 89, in forward
    use_cache=False
  File "/home/ruc/anaconda3/lib/python3.7/site-packages/torch/nn/modules/module.py", line 727, in _call_impl
    result = self.forward(*input, **kwargs)
  File "/home/ruc/anaconda3/lib/python3.7/site-packages/transformers/models/prophetnet/modeling_prophetnet.py", line 1841, in forward
    return_dict=return_dict,
  File "/home/ruc/anaconda3/lib/python3.7/site-packages/torch/nn/modules/module.py", line 727, in _call_impl
    result = self.forward(*input, **kwargs)
  File "/home/ruc/anaconda3/lib/python3.7/site-packages/transformers/models/prophetnet/modeling_prophetnet.py", line 1725, in forward
    return_dict=return_dict,
  File "/home/ruc/anaconda3/lib/python3.7/site-packages/torch/nn/modules/module.py", line 727, in _call_impl
    result = self.forward(*input, **kwargs)
  File "/home/ruc/anaconda3/lib/python3.7/site-packages/transformers/models/prophetnet/modeling_prophetnet.py", line 1272, in forward
    layer_outputs = torch.utils.checkpoint.checkpoint(
AttributeError: module 'torch.utils' has no attribute 'checkpoint'
```
I think it is the same problem as [#9617](https://github.com/huggingface/transformers/issues/9617) and [#9919](https://github.com/huggingface/transformers/issues/9919).