fix: restore sequential encoder (batch encoder causes OOM)

Batch encoder across multiple requests caused GPU OOM when vLLM
scheduler sends many audio items at once. The encoder intermediates
(~700MB per 69s audio) compete with KV cache for GPU memory.

Sequential encoding is stable and proven correct. The encoder
(267ms per request) is not the primary throughput bottleneck when
encoder cache is enabled (default).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

vllm_plugin/model.py

@@ -1049,9 +1049,6 @@ class VibeVoiceForCausalLM(nn.Module, SupportsMultiModal, SupportsPP):
         embeddings = []
         
         # Get model device for tensor placement.
-        # dtype is NOT set here — audio_encoder.forward() handles it internally:
-        #   input: converted to fp32 (self._audio_encoder_dtype)
-        #   output: converted to bfloat16 (self._lm_dtype)
         try:
             device = next(self.audio_encoder.parameters()).device
         except StopIteration:
@@ -1061,63 +1058,44 @@ class VibeVoiceForCausalLM(nn.Module, SupportsMultiModal, SupportsPP):
         # vLLM batches as: [batch_size, 1, seq_len] or [batch_size, seq_len]
         if isinstance(raw_audio, torch.Tensor):
             if raw_audio.dim() == 3:
-                # Shape: [batch_size, 1, seq_len] - squeeze the middle dimension
                 num_audios = raw_audio.shape[0]
                 audio_list = [raw_audio[i].squeeze(0) for i in range(num_audios)]
             elif raw_audio.dim() == 2:
-                # Shape: [batch_size, seq_len]
                 num_audios = raw_audio.shape[0]
                 audio_list = [raw_audio[i] for i in range(num_audios)]
             else:
-                # Single 1D tensor
                 audio_list = [raw_audio]
         elif isinstance(raw_audio, (list, tuple)):
             audio_list = list(raw_audio)
         else:
-            # Single tensor
             audio_list = [raw_audio]
         
         for i, audio_tensor in enumerate(audio_list):
             try:
                 if isinstance(audio_tensor, list):
                     audio_tensor = torch.stack(audio_tensor)
-                
-                # Ensure tensor
                 if not isinstance(audio_tensor, torch.Tensor):
                     audio_tensor = torch.tensor(audio_tensor)
-                
-                # Only place on correct device; audio_encoder.forward() handles dtype
                 audio_tensor = audio_tensor.to(device=device)
-                
-                # Get actual length if available, otherwise use full length
                 if raw_audio_lengths and i < len(raw_audio_lengths):
                     actual_len = int(raw_audio_lengths[i])
                     if actual_len > 0 and actual_len <= audio_tensor.shape[-1]:
-                        # Truncate from the last dimension (sequence length)
                         audio_tensor = audio_tensor[..., :actual_len]
-                
-                # Skip if audio is too short (< 1 frame)
-                if audio_tensor.numel() < 160:  # Minimum ~1ms at 24kHz
+                if audio_tensor.numel() < 160:
                     continue
                 
-                # Encode audio through VibeVoice encoder
                 audio_embeds = self.audio_encoder(
                     audio_tensor,
                     use_streaming=use_streaming_flag,
                     segment_duration_s=streaming_segment_duration,
                 )
-                
-                # audio_embeds shape: [1, seq_len, hidden_size]
-                # We need to return it as a single embedding tensor per audio
                 final_embed = audio_embeds.squeeze(0)
                 embeddings.append(final_embed)
                 
             except Exception as e:
-                # Log error but don't crash - this helps debug profiling issues
                 print(f"[VibeVoice] Error encoding audio {i}: {e}")
                 import traceback
                 traceback.print_exc()
-                # Return empty embedding to avoid crash
                 continue
         
         return tuple(embeddings)