adapter: use rename inside of concatenate transform if appropriate (#264)

Author: paul-buerkner

bayesflow/adapters/adapter.py

@@ -126,12 +126,11 @@ def clear(self):
         self.transforms = []
         return self
 
-    def concatenate(self, keys: Sequence[str], *, into: str, axis: int = -1):
+    def concatenate(self, keys: str | Sequence[str], *, into: str, axis: int = -1):
         if isinstance(keys, str):
-            # this is a common mistake, and also passes the type checker since str is a sequence of characters
-            raise ValueError("Keys must be a sequence of strings. To rename a single key, use the `rename` method.")
-
-        transform = Concatenate(keys, into=into, axis=axis)
+            transform = Rename(keys, to_key=into)
+        else:
+            transform = Concatenate(keys, into=into, axis=axis)
         self.transforms.append(transform)
         return self