add token to codecov

[haifeng-jin]

Resolves #19264
We may need to add the secret to the repo if it is not there yet.
The tests running here are fine.
Let's merge it to see if it works on the master branch.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.36%. Comparing base (c8700f4) to head (0c139a4).
Report is 98 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #19312      +/-   ##
==========================================
- Coverage   80.14%   71.36%   -8.78%     
==========================================
  Files         341      366      +25     
  Lines       36163    40071    +3908     
  Branches     7116     7769     +653     
==========================================
- Hits        28982    28597     -385     
- Misses       5578     9842    +4264     
- Partials     1603     1632      +29     

Flag	Coverage Δ
keras	71.25% <ø> (-8.74%)	⬇️
keras-jax	?
keras-numpy	54.46% <ø> (-2.63%)	⬇️
keras-tensorflow	61.37% <ø> (-3.28%)	⬇️
keras-torch	60.50% <ø> (-3.37%)	⬇️

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[fchollet]

Thanks for the PR. Added it as a "Repository secret".