Support --lora-on-cpu flag for DPO model merging (#2766) [skip ci]

* Support --lora-on-cpu flag for DPO model merging

* fix: use device=cpu in _convert_embedding_modules_dtype when lora_on_cpu is set

src/axolotl/loaders/model.py

@@ -776,6 +776,9 @@ class ModelLoader:
         dist_dtype: torch.dtype,
         before_kbit_train_or_finetune: bool,
     ):
+        dest = {"dtype": dist_dtype}
+        if self.cfg.lora_on_cpu:
+            dest["device"] = "cpu"
         for name, module in self.model.named_modules():
             if "norm" in name:
                 module.to(dist_dtype)
@@ -786,4 +789,4 @@ class ModelLoader:
                     # don't upcast lm_head for btlm
                     continue
             if any(m in name for m in embedding_modules) and hasattr(module, "weight"):
-                module.to(dist_dtype)
+                module.to(**dest)