mel_converter buffers (mel_basis, hann_window) are inference tensors
because the model was loaded inside ComfyUI's torch.inference_mode().
Operations on them propagate the flag to outputs. Clone both target_mel
and pred_mel to get normal autograd-compatible tensors. .clone() is
differentiable so the grad graph to vocoder parameters is preserved.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>