Fix return_dict in encodec
Author: jla524
Pull request: yes

# What does this PR do?

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Fixes #31642 (issue)

With this PR, `return_dict=False` returns a tuple, and the unit test compares tuple vs dict values.

```
% pytest tests/models/encodec/test_modeling_encodec.py -k test_model_outputs_equivalence
=========================================================== test session starts ============================================================
platform darwin -- Python 3.12.3, pytest-7.4.4, pluggy-1.4.0
rootdir: /Users/jacky/repos/transformers
configfile: pyproject.toml
plugins: xdist-3.5.0, timeout-2.3.1, rich-0.1.1
collected 116 items / 115 deselected / 1 selected                                                                                          

tests/models/encodec/test_modeling_encodec.py .                                                                                      [100%]

<warnings redacted>
============================================== 1 passed, 115 deselected, 9 warnings in 1.56s ===============================================
```

## Who can review?

@kamilakesbi

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Comments:
Thanks for iterating on this @jla524! 

@amyeroberts this should be ready for final review/merge :) 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_31646). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.