[pytree] Extract reusable generic tests for pytree (pytorch#110395)

Part of pytorch#109684

- pytorch#109684

Changes:

- Add new functions `tree_structure`, `tree_leaves`, `tree_map_` and `tree_map_only_` to Python pytree.
- Extract reusable tests for pytree to `TestGenericPytree`.
- Change `treespec_dumps` and `treespec_loads` in C++ pytree to call Python pytree and use JSON string as serialization type.
- Rename `torch.utils.pytree` -> `torch.utils._cxx_pytree`.

Pull Request resolved: pytorch#110395
Approved by: https://github.com/zou3519

.lintrunner.toml

@@ -223,7 +223,7 @@ include_patterns = [
     'tools/**/*.py',
     'torchgen/**/*.py',
     'torch/utils/_pytree.py',
-    'torch/utils/pytree.py',
+    'torch/utils/_cxx_pytree.py',
     'torch/utils/benchmark/utils/common.py',
     'torch/utils/benchmark/utils/timer.py',
     'torch/utils/benchmark/utils/valgrind_wrapper/**/*.py',

mypy-strict.ini

@@ -43,7 +43,7 @@ files =
     tools,
     torch/profiler/_memory_profiler.py,
     torch/utils/_pytree.py,
-    torch/utils/pytree.py,
+    torch/utils/_cxx_pytree.py,
     torch/utils/benchmark/utils/common.py,
     torch/utils/benchmark/utils/timer.py,
     torch/utils/benchmark/utils/valgrind_wrapper