Fix keras.ops.softmax for the tensorflow backend

[tirthasheshpatel]

Fixes #19299

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 75.55%. Comparing base (c8700f4) to head (286a89b).
Report is 1183 commits behind head on master.

Files with missing lines	Patch %	Lines
keras/ops/nn.py	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #19300      +/-   ##
==========================================
- Coverage   80.14%   75.55%   -4.59%     
==========================================
  Files         341      365      +24     
  Lines       36163    39861    +3698     
  Branches     7116     7747     +631     
==========================================
+ Hits        28982    30118    +1136     
- Misses       5578     8062    +2484     
- Partials     1603     1681      +78     

Flag	Coverage Δ
keras	75.40% <0.00%> (-4.59%)	⬇️
keras-jax	59.64% <0.00%> (-3.42%)	⬇️
keras-numpy	54.19% <0.00%> (-2.90%)	⬇️
keras-tensorflow	61.15% <0.00%> (-3.51%)	⬇️
keras-torch	60.26% <0.00%> (-3.60%)	⬇️

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[fchollet]

LGTM, thanks!