Revert "Remove future implementation"

This reverts commit 60a21c1.

Author: jackzhxng

examples/models/llama2/export_llama_lib.py

@@ -76,7 +76,7 @@
 
 
 EXECUTORCH_DEFINED_MODELS = ["llama2", "llama3", "llama3_1", "llama3_2"]
-TORCHTUNE_DEFINED_MODELS = []
+TORCHTUNE_DEFINED_MODELS = ["llama3_2_vision"]
 
 
 class WeightType(Enum):
@@ -798,7 +798,8 @@ def _load_llama_model(
         modelname = "llama2"
         model_class_name = "Llama2Model"
     elif modelname in TORCHTUNE_DEFINED_MODELS:
-        raise NotImplementedError("Torchtune Llama models are not yet supported in ExecuTorch export.")
+        if modelname == "llama3_2_vision":
+            model_class_name = "Llama3_2Decoder"
     else:
         raise ValueError(f"{modelname} is not a valid Llama model.")