Fix `_load_pretrained_model`
# What does this PR do ? 
Fixes the following test from my old PR Add check for tied parameters (#24029): 
`RUN_SLOW=1 python3 -m pytest -v tests/models/marian/test_modeling_marian.py::TestMarian_FI_EN_V2::test_batch_generation_en_fr`
This line what something I added in my previous PR thinking that we forgot to tie the weights. But I guess it was done intentionally as I see that one test failed and generated garbage value. Here the error that we get from the test: 
```
tests/models/marian/test_modeling_marian.py:433: in _assert_generated_batch_equal_expected
    self.assertListEqual(self.expected_text, generated_words)
E   AssertionError: Lists differ: ['I like to read books', 'I like watching football'] != ['obliterat obliterat obliterat obliterat o[1345 chars]ɰɰɰ']
E   
E   First differing element 0:
E   'I like to read books'
E   'obliterat obliterat obliterat obliterat o[1214 chars]erat'
E   
E   Diff is 1556 characters long. Set self.maxDiff to None to see it.
```



> Could you explain a bit more why we need to remove this line? Thanks!

<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>BTW, let's make the PR title a bit more precise, sth like `fix _load_pretrained_model` or anything you think more precise, @SunMarc .
Thanks!<|||||>Yeah, I will explore a little bit more why we have this weird behavior before merging.