fix: cast input mel to model dtype before vocoder forward pass

mel_converter outputs float32 (cuFFT requirement) but vocoder weights are
bfloat16 from model loading. Cast input_mel back to model dtype before
feeding the vocoder to avoid conv1d dtype mismatch.

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

nodes/selva_bigvgan_trainer.py

@@ -1036,7 +1036,7 @@ def _do_train(vocoder, mel_converter, clips,
             # ~2x compute for a large reduction in activation memory, allowing
             # batch_size > 1 without OOM.
             pred_wav = torch.utils.checkpoint.checkpoint(
-                vocoder, input_mel, use_reentrant=False
+                vocoder, input_mel.to(dtype), use_reentrant=False
             )                                                     # [B, 1, T_wav]
 
             T = min(pred_wav.shape[-1], target_wav.shape[-1])