feat: Allow loading fine-tuned models in simulstreaming

This change modifies the `simulstreaming` backend to support loading fine-tuned Whisper models via the `--model_dir` argument.

The `SimulStreamingASR` class has been updated to:
- Use the `model_dir` path directly to load the model, which is the correct procedure for fine-tuned `.pt` files.
- Automatically disable the `faster-whisper` and `mlx-whisper` fast encoders when `model_dir` is used, as they are not compatible with standard fine-tuned models.

The call site in `core.py` already passed the `model_dir` argument, so no changes were needed there. This change makes the `simulstreaming` backend more flexible and allows users to leverage their own custom models.

whisperlivekit/simul_whisper/backend.py

@@ -210,11 +210,15 @@ class SimulStreamingASR():
         else:
             self.tokenizer = None
         
-        self.model_name = os.path.basename(self.cfg.model_path).replace(".pt", "")
-        self.model_path = os.path.dirname(os.path.abspath(self.cfg.model_path))
+        if model_dir:
+            self.model_name = model_dir
+            self.model_path = None
+        else:
+            self.model_name = os.path.basename(self.cfg.model_path).replace(".pt", "")
+            self.model_path = os.path.dirname(os.path.abspath(self.cfg.model_path))
     
         self.mlx_encoder, self.fw_encoder = None, None
-        if not self.disable_fast_encoder:
+        if not self.disable_fast_encoder and not model_dir:
             if HAS_MLX_WHISPER:
                 print('Simulstreaming will use MLX whisper for a faster encoder.')
                 mlx_model_name = mlx_model_mapping[self.model_name]