[Roformer] Fixing a bug in RoFormerEncoder where it was ignoring the length of past_key_values when generating as a decoder
Author: TheWall9
Pull request: yes

# What does this PR do?

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This pull request fixes the bug relating to the generation.

When using `RoFormerForCausalLM`, the `RoFormerEncoder` generates position embeddings based on the `seq_len` of the `hidden_states`. However, when using the `generate()` method to generate text token by token, the `seq_len` is always 1 and the length of `past_key_values` is ignored.

Using the solution from https://github.com/JunnYu/RoFormer_pytorch/blob/roformer_v2/src/roformer/modeling_roformer.py

My setup:
>- `transformers` version: 4.28.0.dev0
>- Platform: Windows-10-10.0.19045-SP0
>- Python version: 3.10.10
>- Huggingface_hub version: 0.13.3
>- PyTorch version (GPU?): 2.0.0 (False)

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
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Models:

- text models: @ArthurZucker and @younesbelkada
- vision models: @amyeroberts
- speech models: @sanchit-gandhi
- graph models: @clefourrier

Library:

- flax: @sanchit-gandhi
- generate: @gante
- pipelines: @Narsil
- tensorflow: @gante and @Rocketknight1
- tokenizers: @ArthurZucker
- trainer: @sgugger

Integrations:

- deepspeed: HF Trainer: @stas00, Accelerate: @pacman100
- ray/raytune: @richardliaw, @amogkam

Documentation: @sgugger, @stevhliu and @MKhalusova

HF projects:

- accelerate: [different repo](https://github.com/huggingface/accelerate)
- datasets: [different repo](https://github.com/huggingface/datasets)
- diffusers: [different repo](https://github.com/huggingface/diffusers)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Maintained examples (not research project or legacy):

- Flax: @sanchit-gandhi
- PyTorch: @sgugger
- TensorFlow: @Rocketknight1

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@JunnYu 
@ArthurZucker
@younesbelkada

Comments:
_The documentation is not available anymore as the PR was closed or merged._

Congrats for this first PR 🔥 And thanks for contributing 🤗