fix(bigvgan-trainer): resolve device mismatch in _save_sample after offload

After the finally block, offload_to_cpu moves the vocoder to CPU while
ref_mel stays on GPU. Fix: detect vocoder's current device via
next(vocoder.parameters()).device and move ref_mel there before vocoding.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nodes/selva_bigvgan_trainer.py

@@ -182,8 +182,11 @@ class SelvaBigvganTrainer:
         def _save_sample(label):
             """Vocode the reference mel and save as .wav."""
             try:
+                # Vocoder may have been offloaded to CPU after training — match its device.
+                voc_device = next(vocoder.parameters()).device
+                mel = ref_mel.to(voc_device)
                 with torch.no_grad():
-                    wav = vocoder(ref_mel)          # [1, 1, T] or [1, T]
+                    wav = vocoder(mel)              # [1, 1, T] or [1, T]
                     if wav.dim() == 2:
                         wav = wav.unsqueeze(1)
                     wav = wav.float().cpu().clamp(-1, 1)