[BE] bump optree version to 0.12.1 (#130139)

0.12.0 Major Updates:

- Add context manager to temporarily set the dictionary sorting mode
- Add accessor APIs
- Use `stable` tag for `pybind11` for Python 3.13 support
- Fix potential segmentation fault for pickling support

0.12.1 Updates:

- Fix warning regression during import when launch with strict warning filters

Closes #130155
Pull Request resolved: https://github.com/pytorch/pytorch/pull/130139
Approved by: https://github.com/zou3519

torch/utils/_cxx_pytree.py

@@ -273,7 +273,7 @@ def tree_flatten(
     >>> from collections import OrderedDict
     >>> tree = OrderedDict([('b', (2, [3, 4])), ('a', 1), ('c', None), ('d', 5)])
     >>> tree_flatten(tree)
-    ([2, 3, 4, 1, None, 5], PyTreeSpec(OrderedDict([('b', (*, [*, *])), ('a', *), ('c', *), ('d', *)]), NoneIsLeaf))
+    ([2, 3, 4, 1, None, 5], PyTreeSpec(OrderedDict({'b': (*, [*, *]), 'a': *, 'c': *, 'd': *}), NoneIsLeaf))
 
     Args:
         tree (pytree): A pytree to flatten.