fix(musicgen_melody): use DynamicCache instead of EncoderDecoderCache

[adityachoksi2512]

What does this PR do?

Fixes #45647

MusicgenMelody fuses encoder_hidden_states directly into inputs_embeds
and uses pure self-attention — it does not use cross-attention. Using
EncoderDecoderCache caused audio conditioning to be silently ignored
during generation, producing byte-identical output regardless of the audio input.

Root cause

Identified via git bisect — the regression was introduced in #38635, which
refactored the cache system. The decoder was incorrectly initialized with
EncoderDecoderCache when it only needs a plain DynamicCache.

Fix

One line change in MusicgenMelodyDecoder.forward():

# Before
past_key_values = EncoderDecoderCache(DynamicCache(config=self.config), DynamicCache(config=self.config))

# After  
past_key_values = DynamicCache(config=self.config)

Testing

Existing test suite passes (133 passed, 62 skipped).
Regression test for this specific bug is in #45737.

[github-actions]

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

run-slow: musicgen_melody

[adityachoksi2512]

I noticed @voodoovampire removed the regression test with a note about deeper investigation. Happy to revisit the fix if the root cause turns out to be more complex - let me know if additional changes are needed. cc @ebezzam @eustlb

[voodoovampire]

Hey @adityachoksi2512 - just to clarify the situation:

I independently created PR #45737 which includes:

1.Your cache fix (cherry-picked with full co-author credit to you)

2.A regression test that proves the audio conditioning bug still exists even with the cache fix

I temporarily removed the test but have now restored it as @pytest.mark.xfail to document the expected behavior for future work.

Your cache fix prevents crashes, which is valuable. But the regression test shows audio conditioning is still broken - two different audio inputs produce identical outputs. The root cause needs deeper investigation beyond just the cache type change.

Both PRs address the same issue - maintainers will decide which to merge. Just wanted to make sure the full context is clear.

[adityachoksi2512]

Thanks for the clarification @voodoovampire. Good point on the deeper investigation - I'll defer to the maintainers on next steps. Happy to contribute further in whatever direction they suggest. cc @ebezzam @eustlb