Fix implicit import of flax.struct

[copybara-service]

Fix implicit import of flax.struct

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[codecov-commenter]

Codecov Report

Merging #3480 (991d3b7) into main (2f6e5ff) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #3480   +/-   ##
=======================================
  Coverage   52.10%   52.11%           
=======================================
  Files          91       91           
  Lines       10968    10969    +1     
=======================================
+ Hits         5715     5716    +1     
  Misses       5253     5253           

Files	Coverage Δ
flax/__init__.py	100.00% <100.00%> (ø)