The goal of this PR is to provide a standard way to create simple treespec instances and hide the implementation details of the `PyTreeSpec` class.
Changes:
1. Add function `treespec_leaf()` to replace `LeafSpec()`.
2. Add function `treespec_tuple(...)` and `treespec_dict(...)` to create treespec for `tuple` / `dict` which is used for `*args` / `**kwargs`. This avoids direct modification to `treespec` instances that rely on the implementation details of the `PyTreeSpec` class.
3. Change `len(spec.children_specs)` to `spec.num_children`.
4. Change `isinstance(spec, LeafSpec)` to `spec.is_leaf()`.
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Pull Request resolved: https://github.com/pytorch/pytorch/pull/160843
Approved by: https://github.com/mlazos
e8fadba28c