fix: restore dtype after float32 STFT in discriminator spectrogram

torch.stft requires float32 input, but the .float() cast was not
reversed before the spectrogram hit bfloat16 Conv2d weights. Save
the original dtype and cast back after abs().

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

nodes/selva_bigvgan_trainer.py

@@ -125,8 +125,9 @@ class _DiscriminatorR(nn.Module):
         x = x.squeeze(1)  # [B, T]
         pad = (win - hop) // 2
         x = F.pad(x, (pad, pad + (win - hop) % 2), mode="reflect")
+        orig_dtype = x.dtype
         x = torch.stft(x.float(), n, hop, win, window, center=False, return_complex=True)
-        x = x.abs().unsqueeze(1)  # [B, 1, freq, time]
+        x = x.abs().to(orig_dtype).unsqueeze(1)  # [B, 1, freq, time]
         return x
 
     def forward(self, x):