[Bugfix] Fix shape mismatch in LongCatAudioDiTTransformer conversion

[RuixiangMa]

Fixes # (issue)

RuntimeError: Error(s) in loading state_dict for LongCatAudioDiTTransformer:
        size mismatch for blocks.0.ffn.ff.0.weight: copying a param with shape torch.Size([9216, 2560]) from checkpoint, the shape in current model is torch.Size([10240, 2560]).

Root cause:
The ff_mult parameter was hardcoded to 4.0 in the transformer but the LongCat-AudioDiT-3.5B checkpoint uses ff_mult=3.6, causing FFN weight shape mismatch (9216 vs 10240).

Now reads dit_ff_mult from config.json, enabling conversion of models with non-default ff_mult (e.g., 3.5B model uses 3.6).

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[dg845]

Thanks for the PR!

[dg845]

Merging as the CI failures are unrelated.