Fix whisper return_language with return_timestamp=word

[Metric-Void]

What does this PR do?

Fixes #39404.

Add a switch to Whisper.generate() that allows preserving some special tokens, then stripped in retrieve_segments to ensure timestamp alignment.

Tested on short and long audios. Tested on English, French, and Cantonese. Prediction and timestamp results align, and language is detected correctly.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
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  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@eustlb @ebezzam

Local failed tests (WSL2, RUN_SLOW)

$ pytest tests/models/whisper
================================================================================================================= short test summary info ==================================================================================================================
FAILED tests/models/whisper/test_modeling_whisper.py::WhisperModelTest::test_flex_attention_with_grads - torch._inductor.exc.InductorError: LoweringException: CalledProcessError: Command '['/usr/bin/gcc', '/tmp/tmpw7mv95z8/main.c', '-O3', '-shared', '-fPIC', '-Wno-psabi', '-o', '/tmp/tmpw7mv95z8/cuda_utils.cpython-312-x86_64-linux-gnu.so', '-lcuda', ...
FAILED tests/models/whisper/test_modeling_whisper.py::WhisperModelTest::test_sdpa_can_compile_dynamic - torch._inductor.exc.InductorError: CalledProcessError: Command '['/usr/bin/gcc', '/tmp/tmp2jbthzzq/main.c', '-O3', '-shared', '-fPIC', '-Wno-psabi', '-o', '/tmp/tmp2jbthzzq/cuda_utils.cpython-312-x86_64-linux-gnu.so', '-lcuda', '-L/home/metricvoid...
FAILED tests/models/whisper/test_modeling_whisper.py::WhisperEncoderModelTest::test_flex_attention_with_grads - torch._inductor.exc.InductorError: LoweringException: CalledProcessError: Command '['/usr/bin/gcc', '/tmp/tmpbszajy61/main.c', '-O3', '-shared', '-fPIC', '-Wno-psabi', '-o', '/tmp/tmpbszajy61/cuda_utils.cpython-312-x86_64-linux-gnu.so', '-lcuda', ...
FAILED tests/models/whisper/test_modeling_whisper.py::WhisperEncoderModelTest::test_sdpa_can_compile_dynamic - torch._inductor.exc.InductorError: CalledProcessError: Command '['/usr/bin/gcc', '/tmp/tmpevr_eml0/main.c', '-O3', '-shared', '-fPIC', '-Wno-psabi', '-o', '/tmp/tmpevr_eml0/cuda_utils.cpython-312-x86_64-linux-gnu.so', '-lcuda', '-L/home/metricvoid...
FAILED tests/models/whisper/test_modeling_whisper.py::WhisperStandaloneDecoderModelTest::test_generate_compilation_all_outputs - torch._inductor.exc.InductorError: CalledProcessError: Command '['/usr/bin/gcc', '/tmp/tmpghb4htrw/main.c', '-O3', '-shared', '-fPIC', '-Wno-psabi', '-o', '/tmp/tmpghb4htrw/cuda_utils.cpython-312-x86_64-linux-gnu.so', '-lcuda', '-L/home/metricvoid...
FAILED tests/models/whisper/test_modeling_whisper.py::WhisperStandaloneDecoderModelTest::test_generate_compile_model_forward - torch._inductor.exc.InductorError: CalledProcessError: Command '['/usr/bin/gcc', '/tmp/tmpb3fj6t8c/main.c', '-O3', '-shared', '-fPIC', '-Wno-psabi', '-o', '/tmp/tmpb3fj6t8c/cuda_utils.cpython-312-x86_64-linux-gnu.so', '-lcuda', '-L/home/metricvoid...
FAILED tests/models/whisper/test_modeling_whisper.py::WhisperStandaloneDecoderModelTest::test_generate_from_inputs_embeds_with_static_cache - torch._inductor.exc.InductorError: CalledProcessError: Command '['/usr/bin/gcc', '/tmp/tmp122w6v5o/main.c', '-O3', '-shared', '-fPIC', '-Wno-psabi', '-o', '/tmp/tmp122w6v5o/cuda_utils.cpython-312-x86_64-linux-gnu.so', '-lcuda', '-L/home/metricvoid...
FAILED tests/models/whisper/test_modeling_whisper.py::WhisperStandaloneDecoderModelTest::test_generate_with_static_cache - torch._inductor.exc.InductorError: CalledProcessError: Command '['/usr/bin/gcc', '/tmp/tmpee6hyznt/main.c', '-O3', '-shared', '-fPIC', '-Wno-psabi', '-o', '/tmp/tmpee6hyznt/cuda_utils.cpython-312-x86_64-linux-gnu.so', '-lcuda', '-L/home/metricvoid...
FAILED tests/models/whisper/test_modeling_whisper.py::WhisperStandaloneDecoderModelTest::test_sdpa_can_compile_dynamic - torch._inductor.exc.InductorError: CalledProcessError: Command '['/usr/bin/gcc', '/tmp/tmpbz2lnr80/main.c', '-O3', '-shared', '-fPIC', '-Wno-psabi', '-o', '/tmp/tmpbz2lnr80/cuda_utils.cpython-312-x86_64-linux-gnu.so', '-lcuda', '-L/home/metricvoid...
FAILED tests/models/whisper/test_tokenization_whisper.py::WhisperTokenizerTest::test_padding_side_in_kwargs - ImportError: 
FAILED tests/models/whisper/test_tokenization_whisper.py::WhisperTokenizerTest::test_tokenizer_initialization_with_conflicting_key - ImportError: 
FAILED tests/models/whisper/test_tokenization_whisper.py::WhisperTokenizerTest::test_tokenizer_mismatch_warning - ImportError: 
FAILED tests/models/whisper/test_tokenization_whisper.py::WhisperTokenizerTest::test_truncation_side_in_kwargs - ImportError: 
=========================================================================================== 13 failed, 445 passed, 295 skipped, 36 warnings in 166.72s (0:02:46) ===========================================================================================

I don't think any of these failures are related to this PR.

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: whisper