feat: add no_rope support for residual LM and fix streaming continuation decoding

- Add `residual_lm_no_rope` config option in VoxCPMConfig and propagate to MiniCPMModel
- Add `no_rope` field to MiniCPM4Config; make RoPE embedding optional in MiniCPMModel and MiniCPMAttention
- Add `streaming_prefix_len` parameter to generation interface
- Fix non-streaming audio decode in continuation mode to trim leading prefix patches consistently
- Refactor streaming prefix context preparation: distinguish continuation vs. zero-shot via feat_mask trailing bit instead of audio_mask sum

Made-with: Cursor

src/voxcpm/model/voxcpm2.py

@@ -129,6 +129,7 @@ class VoxCPMConfig(BaseModel):
     patch_size: int = 4
     feat_dim: int = 64
     residual_lm_num_layers: int = 8
+    residual_lm_no_rope: bool = False
     scalar_quantization_latent_dim: int = 512
     scalar_quantization_scale: int = 9
 
@@ -195,6 +196,7 @@ class VoxCPM2Model(nn.Module):
         residual_lm_config = config.lm_config.model_copy(deep=True)
         residual_lm_config.num_hidden_layers = config.residual_lm_num_layers
         residual_lm_config.vocab_size = 0
+        residual_lm_config.no_rope = config.residual_lm_no_rope
         self.residual_lm = MiniCPMModel(residual_lm_config)
         self.residual_lm.setup_cache(1, config.max_length, self.device, get_dtype(self.config.dtype))
 
@@ -474,6 +476,7 @@ class VoxCPM2Model(nn.Module):
         retry_badcase_max_times: int = 3,
         retry_badcase_ratio_threshold: float = 6.0,
         streaming: bool = False,
+        streaming_prefix_len: int = 3,
     ) -> Generator[torch.Tensor, None, None]:
         if retry_badcase and streaming:
             warnings.warn("Retry on bad cases is not supported in streaming mode, setting retry_badcase=False.")
@@ -634,6 +637,7 @@ class VoxCPM2Model(nn.Module):
                 inference_timesteps=inference_timesteps,
                 cfg_value=cfg_value,
                 streaming=streaming,
+                streaming_prefix_len=streaming_prefix_len,
             )
             if streaming:
                 patch_len = self.patch_size * self.chunk_size
@@ -658,7 +662,13 @@ class VoxCPM2Model(nn.Module):
                     break
 
         if not streaming:
-            decode_audio = self.audio_vae.decode(latent_pred.to(torch.float32)).squeeze(1).cpu()
+            decode_audio = self.audio_vae.decode(latent_pred.to(torch.float32))
+            patch_len = self.patch_size * self.chunk_size
+            has_continuation = bool(prompt_wav_path)
+            if has_continuation:
+                decode_audio = decode_audio[..., patch_len * (streaming_prefix_len - 1):].squeeze(1).cpu()
+            else:
+                decode_audio = decode_audio.squeeze(1).cpu()
             yield decode_audio
 
     @torch.inference_mode()
@@ -930,7 +940,7 @@ class VoxCPM2Model(nn.Module):
         if not streaming:
             decode_audio = self.audio_vae.decode(latent_pred.to(torch.float32))
             patch_len = self.patch_size * self.chunk_size
-            if audio_mask.sum().item() > 0:
+            if mode in ("continuation", "ref_continuation"):
                 decode_audio = decode_audio[..., patch_len * (streaming_prefix_len - 1) :].squeeze(1).cpu()
             else:
                 decode_audio = decode_audio[..., :].squeeze(1).cpu()
@@ -995,15 +1005,17 @@ class VoxCPM2Model(nn.Module):
         curr_embed = None
 
         # Prepare prompt context patches for streaming mode
-        # When there's a prompt audio, use its last (streaming_prefix_len - 1) patches as initial context
-        prompt_context_patches = []
-        audio_patch_count = int(feat_mask.sum().item())
-        if audio_patch_count > 0:
-            context_len = min(streaming_prefix_len - 1, audio_patch_count)
+        # - Continuation modes (feat_mask ends with 1): use the last (streaming_prefix_len - 1)
+        #   trailing audio patches as initial context so the VAE can decode smoothly.
+        # - Reference-only / zero-shot (feat_mask ends with 0): start from scratch.
+        has_continuation_audio = feat_mask[0, -1].item() == 1
+        if has_continuation_audio:
             audio_indices = feat_mask.squeeze(0).nonzero(as_tuple=True)[0]
+            context_len = min(streaming_prefix_len - 1, len(audio_indices))
             last_audio_indices = audio_indices[-context_len:]
-            prompt_context_patches = list(feat[:, last_audio_indices, :, :].split(1, dim=1))
-            pred_feat_seq = prompt_context_patches + pred_feat_seq
+            pred_feat_seq = list(feat[:, last_audio_indices, :, :].split(1, dim=1))
+        else:
+            pred_feat_seq = []
 
         enc_outputs, kv_cache_tuple = self.base_lm(
             inputs_embeds=combined_embed,