fix: cast mel_converter buffers to float32 to match STFT input dtype

mel_basis and hann_window buffers inherit bfloat16 from model loading.
Since all mel_converter inputs are cast to float32 for cuFFT, the
internal buffers must also be float32 to avoid matmul dtype mismatch.

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

nodes/selva_bigvgan_trainer.py

@@ -808,9 +808,11 @@ def _do_train(vocoder, mel_converter, clips,
     clips = [c.clone() for c in clips]
 
     # 2. mel_converter buffers (mel_basis, hann_window) — same origin.
+    #    Also cast to float32: mel_converter receives float32 audio (cuFFT
+    #    requirement) so all internal buffers must match.
     for name, buf in list(mel_converter._buffers.items()):
         if buf is not None:
-            mel_converter._buffers[name] = buf.clone()
+            mel_converter._buffers[name] = buf.clone().float()
 
     # 3. Vocoder parameters are handled below with clone().detach().
     # ─────────────────────────────────────────────────────────────────────────