validate checkpoint is consistent with meta_to_tune flag

Summary:
add a flag that do validation when load_checkpoint passed unexpected meta_to_tune flag.
i.e. if this flag is true but checkpoint is not in meta format, or
if this flag is flase but checkpoint is in meta format
Do validation early to avoid unexpected error later.

Differential Revision: D74784778

Author: lijia19

torchtune/models/convert_weights.py

@@ -72,6 +72,21 @@ def get_mapped_key(key: str, mapping_dict: Dict[str, str]) -> str:
 
     return new_key
 
+def is_in_meta_format(state_dict: Dict[str, torch.Tensor]) -> bool:
+    """
+    Check whether the state dict is in Meta's format by checking the presence
+    of unique keys only available in META format.
+    """
+    unique_meta_keys = {k for k, v in _FROM_META.items() if k != v}
+    for key in state_dict.keys():
+        if key not in ["rope.freqs"]:  # Skip loading the position embeddings
+            if any(k.isdigit() for k in key.split(".")):
+                # Replace layer number with "{}" to create key for lookup
+                key = re.sub(r"(\.\d+)", ".{}", key)
+            if key in unique_meta_keys:
+                return True
+    return False
+
 
 def meta_to_tune(state_dict: Dict[str, torch.Tensor]) -> Dict[str, torch.Tensor]:
     """