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param.data.clone() and tensor.detach() on inference tensors both produce inference tensors — the flag propagates through all operations on them. Inside inference_mode(False), torch.zeros() creates genuine normal tensors. Use zeros+copy_ to sanitize both vocoder parameters and mel_converter buffers once before training, so autograd can save inputs for backward. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>