[Feature Request] Add constructors to create `PyTreeSpec` instances on Python side

[XuehaiPan]

Required prerequisites

• [X] I have searched the Issue Tracker that this hasn't already been reported. (comment there if it has.)

Motivation

Currently, we can only create PyTreeSpec instances via optree.tree_flatten(...), optree.tree_structure(...), or similar APIs. It requires the user to get a pytree first and then flatten it.

Now we only have:

• treespec_leaf
• treespec_none
• treespec_tuple

functions to create PyTreeSpec instances via its children treespecs. Some primitive Python types (e.g., list, dict) are not supported yet.

Solution

Add more constructor functions, e.g.:

• treespec_list
• treespec_dict
• treespec_namedtuple
• treespec_ordereddict
• treespec_defaultdict
• treespec_deque
• treespec_structseq
• treespec_custom

Alternatives

No response

Additional context

No response