Logging KT's key order warning only once (pytorch#2548)

Summary:

this warning is very noisy, changing to print it only once instead of every time

Reviewed By: TroyGarden

Differential Revision: D65700079

torchrec/sparse/jagged_tensor.py

@@ -3380,14 +3380,21 @@ def _kt_unflatten(
     return KeyedTensor(context[0], context[1], values[0])
 
 
+print_flatten_spec_warn = True
+
+
 def _kt_flatten_spec(kt: KeyedTensor, spec: TreeSpec) -> List[torch.Tensor]:
     _keys, _length_per_key = spec.context
     #  please read https://fburl.com/workplace/8bei5iju for more context,
     #  you can also consider use short_circuit_pytree_ebc_regroup with KTRegroupAsDict
-    logger.warning(
-        "KT's key order might change from spec from the torch.export, this could have perf impact. "
-        f"{kt.keys()} vs {_keys}"
-    )
+    global print_flatten_spec_warn
+    if print_flatten_spec_warn:
+        logger.warning(
+            "KT's key order might change from spec from the torch.export, this could have perf impact. "
+            f"{kt.keys()} vs {_keys}"
+        )
+        print_flatten_spec_warn = False
+
     res = permute_multi_embedding([kt], [_keys])
     return [res[0]]