[flax] Fix a string formatting bug, and some other minor issues.

[copybara-service]

[flax] Fix a string formatting bug, and some other minor issues.

[google-cla]

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[codecov-commenter]

Codecov Report

Merging #1375 (381c9ba) into master (87a2111) will not change coverage.
The diff coverage is 40.00%.

@@           Coverage Diff           @@
##           master    #1375   +/-   ##
=======================================
  Coverage   82.29%   82.29%           
=======================================
  Files          65       65           
  Lines        5322     5322           
=======================================
  Hits         4380     4380           
  Misses        942      942           

Impacted Files	Coverage Δ
flax/serialization.py	90.06% <40.00%> (ø)

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[google-cla]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

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[google-cla]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

ℹ️ Googlers: Go here for more info.