PegasusForConditionalGeneration - Error in loading state dictionary
## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 3.0.2
- Platform: Linux-5.3.0-1034-azure-x86_64-with-debian-buster-sid
- Python version: 3.7.7
- PyTorch version (GPU?): 1.6.0 (True)
- Tensorflow version (GPU?): not installed
- Using GPU in script?: Tried both
- Using distributed or parallel set-up in script?: No

### Who can help
@sshleifer

## Information

Model I am using (Bert, XLNet ...): google/pegasus-arxiv

The problem arises when using:
the official example scripts: 

The tasks I am working on is:
generating summary using pegasus-arxiv

## To reproduce

Steps to reproduce the behavior:
run the below script
```ruby
mname = "google/pegasus-arxiv"
model = PegasusForConditionalGeneration.from_pretrained(mname)
```

This is throwing error
 File "abstractive_summarizer.py", line 21, in <module>
    model = PegasusForConditionalGeneration.from_pretrained(mname, force_download=True)
  File "/anaconda/envs/py37_default/lib/python3.7/site-packages/transformers/modeling_utils.py", line 894, in from_pretrained
    model.__class__.__name__, "\n\t".join(error_msgs)
RuntimeError: Error(s) in loading state_dict for PegasusForConditionalGeneration:
	size mismatch for model.encoder.embed_positions.weight: copying a param with shape torch.Size([512, 1024]) from checkpoint, the shape in current model is torch.Size([1024, 1024]).
	size mismatch for model.decoder.embed_positions.weight: copying a param with shape torch.Size([512, 1024]) from checkpoint, the shape in current model is torch.Size([1024, 1024]).

## Expected behavior
I tried running a sample in console this morning and it worked fine and I was able to generate summary using pegasus-arxiv. Once I transferred this to Jupyter notebook for some trial purpose, it downloaded pegasus-arxiv and after the same has been giving this error.

(If not able to simulate, please try with force_download=True)

I just fixed. Can you try again. Should produce a warning but no error.<|||||>> I just fixed. Can you try again. Should produce a warning but no error.

I am still getting the below error. It does not seem to even go to the tokenizer. It throws the error right when we acquire the pretrained model. It seems as though something about the checkpoint of the pre-trained model has changed

RuntimeError: Error(s) in loading state_dict for PegasusForConditionalGeneration:
	size mismatch for model.encoder.embed_positions.weight: copying a param with shape torch.Size([512, 1024]) from checkpoint, the shape in current model is torch.Size([1024, 1024]).
	size mismatch for model.decoder.embed_positions.weight: copying a param with shape torch.Size([512, 1024]) from checkpoint, the shape in current model is torch.Size([1024, 1024]).<|||||>I changed the state dict and deleted `model.encoder.embed_positions.weight` at 6pm EST.

Your code works for me on master (93c5c9a5) 

command: `transformers-cli env`

```
- `transformers` version: 3.0.2
- Platform: Darwin-19.5.0-x86_64-i386-64bit
- Python version: 3.7.7
- PyTorch version (GPU?): 1.5.1 (False)
- Tensorflow version (GPU?): 2.2.0 (False)
- Using GPU in script?: <fill in>
- Using distributed or parallel set-up in script?: <fill in>
```
<|||||>> `model.encoder.embed_positions.weight`

I completely removed transformers and took a fresh git and I am still getting the same error. Is there some other information I could provide that will help resolve this issue for me?<|||||>> I changed the state dict and deleted `model.encoder.embed_positions.weight` at 6pm EST.
> 
> Your code works for me on master ([93c5c9a](https://github.com/huggingface/transformers/commit/93c5c9a528475db73c2b481131578b8dd903efba))
> 
> command: `transformers-cli env`
> 
> ```
> - `transformers` version: 3.0.2
> - Platform: Darwin-19.5.0-x86_64-i386-64bit
> - Python version: 3.7.7
> - PyTorch version (GPU?): 1.5.1 (False)
> - Tensorflow version (GPU?): 2.2.0 (False)
> - Using GPU in script?: <fill in>
> - Using distributed or parallel set-up in script?: <fill in>
> ```

The error occurs only for pegasus-arxiv. It works with warning for pegasus-pubmed and pegasus-large. I need help with pegasus-arxiv.
