Ambiguous positional embedding management in LongformerEmbeddings
### System Info

```shell
Current main version of the transformer lib (4.20.1?)
```


### Who can help?

@ydshieh

### Information

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

### Tasks

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

### Reproduction

Hi!

In the code of the LongformerEmbeddings ([here](https://github.com/huggingface/transformers/blob/401fcca6c561d61db6ce25d9b1cebb75325a034f/src/transformers/models/longformer/modeling_longformer.py#L459)), there is unreachable code for the `position_ids` in `forward`.

```python
   def forward(self, input_ids=None, token_type_ids=None, position_ids=None, inputs_embeds=None):
        if position_ids is None:
            if input_ids is not None:
                # Create the position ids from the input token ids. Any padded tokens remain padded.
                position_ids = create_position_ids_from_input_ids(input_ids, self.padding_idx).to(input_ids.device)
            else:
                position_ids = self.create_position_ids_from_inputs_embeds(inputs_embeds)

        ...

        # Here, code is unreachable
        # as both create_position_ids_from_input_ids and create_position_ids_from_inputs_embeds always return stg 
        if position_ids is None:
            position_ids = self.position_ids[:, :seq_length]
```

So, what is the actual expected behavior of this layer for the positional embeddings ids? 
Is it supposed to use self.position_ids or not? 
(If yes, then this is indeed a bug, if not, then some code could be removed for clarity)

Hi @clefourrier 

Looking the PR #7352, `LongformerEmbeddings` was originally using (or being) `RobertaEmbeddings` at that time, and both had the line `if position_ids is None:`.

Looking current `RobertaEmbeddings`, there is no more such line. So I think we can remove it too for `LongformerEmbeddings` without any doubt (actually, it is already very obvious, but I just to find more evidence 😄 )

Would you like to open a PR?


