fix: fix optree compatibility for multi-tree-map with None values

[XuehaiPan]

Description

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Fix None handling for optimizers.

Motivation and Context

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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

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Types of changes

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• 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)
• Example (update in the folder of example)

Checklist

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• 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)
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[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (86b167c) 93.65% compared to head (7f63f10) 93.69%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #195      +/-   ##
==========================================
+ Coverage   93.65%   93.69%   +0.04%     
==========================================
  Files          83       83              
  Lines        2945     2963      +18     
==========================================
+ Hits         2758     2776      +18     
  Misses        187      187              

Flag	Coverage Δ
unittests	93.69% <97.78%> (+0.04%)	⬆️

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

Files	Coverage Δ
torchopt/alias/sgd.py	100.00% <ø> (ø)
torchopt/alias/utils.py	98.84% <100.00%> (+0.06%)	⬆️
torchopt/linalg/cg.py	98.15% <ø> (ø)
torchopt/linalg/ns.py	41.38% <ø> (ø)
torchopt/transform/add_decayed_weights.py	95.71% <100.00%> (+0.13%)	⬆️
torchopt/transform/scale_by_adadelta.py	100.00% <100.00%> (ø)
torchopt/transform/scale_by_adam.py	100.00% <100.00%> (ø)
torchopt/transform/scale_by_adamax.py	100.00% <100.00%> (ø)
torchopt/transform/scale_by_rms.py	100.00% <100.00%> (ø)
torchopt/transform/scale_by_rss.py	100.00% <100.00%> (ø)
... and 6 more

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[Benjamin-eecs]

Please fix lint error.

[Benjamin-eecs]

LGTM!