Use defaultdict (#307)

Co-authored-by: Chunyang Wen <chunyang_wen@apple.com>

python/mlx/utils.py

@@ -1,5 +1,7 @@
 # Copyright © 2023 Apple Inc.
 
+from collections import defaultdict
+
 
 def tree_map(fn, tree, *rest, is_leaf=None):
     """Applies ``fn`` to the leaves of the python tree ``tree`` and
@@ -128,12 +130,10 @@ def tree_unflatten(tree):
         is_list = False
 
     # collect children
-    children = {}
+    children = defaultdict(list)
     for key, value in tree:
         current_idx, *next_idx = key.split(".", maxsplit=1)
         next_idx = "" if not next_idx else next_idx[0]
-        if current_idx not in children:
-            children[current_idx] = []
         children[current_idx].append((next_idx, value))
 
     # recursively map them to the original container