fix(examples/implicit): fix iMAML example with functional APIs

[XuehaiPan]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 81.48148% with 5 lines in your changes missing coverage. Please review.

Project coverage is 65.20%. Comparing base (920bc4a) to head (31da6f1).
Report is 141 commits behind head on main.

Files	Patch %	Lines
torchopt/utils.py	0.00%	4 Missing ⚠️
torchopt/optim/meta/base.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #108   +/-   ##
=======================================
  Coverage   65.20%   65.20%           
=======================================
  Files          62       62           
  Lines        2652     2655    +3     
=======================================
+ Hits         1729     1731    +2     
- Misses        923      924    +1     

Flag	Coverage Δ
unittests	65.20% <81.48%> (+<0.01%)	⬆️

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