feat(ops): add tree flatten and tree map functions with extra paths

[XuehaiPan]

Description

Describe your changes in detail.

Motivation and Context

Why is this change required? What problem does it solve?
If it fixes an open issue, please link to the issue here.
You can use the syntax close #15213 if this solves the issue #15213

• I have raised an issue to propose this change (required for new features and bug fixes)

Add functions tree_flatten_with_path, tree_paths, and tree_map_with_path.

The path is a tuple of entries (indices or keys) to the corresponding leaf. The depth of the leaf can be derivated by depth = len(path).

paths, leaves, treespec = optree.tree_flatten_with_path(tree)
paths = optree.tree_paths(tree)

def fn(p, x, *xs):
    ...

mapped_tree = optree.tree_map_with_path(fn, tree, *rests)

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds core functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)

Checklist

Go over all the following points, and put an x in all the boxes that apply.
If you are unsure about any of these, don't hesitate to ask. We are here to help!

• I have read the CONTRIBUTION guide. (required)
• My change requires a change to the documentation.
• I have updated the tests accordingly. (required for a bug fix or a new feature)
• I have updated the documentation accordingly.
• I have reformatted the code using make format. (required)
• I have checked the code using make lint. (required)
• I have ensured make test pass. (required)

[codecov-commenter]

Codecov Report

Base: 92.65% // Head: 92.93% // Increases project coverage by +0.28% 🎉

Coverage data is based on head (f233607) compared to base (582f355).
Patch coverage: 90.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #11      +/-   ##
==========================================
+ Coverage   92.65%   92.93%   +0.28%     
==========================================
  Files           4        4              
  Lines         245      269      +24     
==========================================
+ Hits          227      250      +23     
- Misses         18       19       +1     

Flag	Coverage Δ
unittests	92.93% <90.00%> (+0.28%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
optree/__init__.py	100.00% <ø> (ø)
optree/registry.py	95.50% <ø> (ø)
optree/ops.py	90.62% <90.00%> (+0.91%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.