fix(aero_realtime): fix NaN loss and duplicate system prompts

- Make time_embedding.inv_freq persistent=True so it survives FSDP2
  state dict save/load cycle (was corrupted → NaN loss)
- Remove auto-injected system prompt from chat template that was
  added before every turn due to per-message apply_chat_template

Author: kcz358

src/lmms_engine/datasets/processor/aero_realtime_processor.py

@@ -615,9 +615,6 @@ def chat_template(self):
             "{% set image_count = namespace(value=0) %}"
             "{% set video_count = namespace(value=0) %}"
             "{% for message in messages %}"
-                "{% if loop.first and message['role'] != 'system' %}"
-                    "<|im_start|>system\nYou are a helpful assistant.<|im_end|>\n"
-                "{% endif %}"
                 "<|im_start|>{{ message['role'] }}\n"
                 "{% if message['content'] is string %}"
                     "{{ message['content'] }}<|im_end|>\n"

src/lmms_engine/models/aero_realtime/modeling_aero_realtime.py

@@ -138,7 +138,7 @@ def __init__(self, dim: int, theta: float = 10000.0) -> None:
         inv_freq = torch.exp(
             -math.log(self.theta) * torch.arange(self.dim // 2).float() / (self.dim // 2)
         )
-        self.register_buffer("inv_freq", inv_freq, persistent=False)
+        self.register_buffer("inv_freq", inv_freq, persistent=True)
 
     def forward(self, time_tensor: torch.Tensor) -> torch.Tensor:
         inv_freq = self.inv_freq.to(device=time_tensor.device, dtype=time_tensor.dtype)