add Module.module_paths

[cgarciae]

What does this PR do?

Adds a Module.module_paths method that returns a mapping between paths and copies of unbound Module instances. module_paths behaves similarly to tabulate, they both use the same machinery from the summary module.

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (70abeef) 56.25% compared to head (a638b16) 56.25%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
flax/linen/module.py	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3654      +/-   ##
==========================================
- Coverage   56.25%   56.25%   -0.01%     
==========================================
  Files         101      101              
  Lines       12177    12183       +6     
==========================================
+ Hits         6850     6853       +3     
- Misses       5327     5330       +3     

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