Fix the Bug in func preprocess_input when x in 3D and data_format=='channels_first'

[ILCSFNO]

Fixes #21749

[gemini-code-assist]

Summary of Changes

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This pull request resolves an issue within the preprocess_input utility function in Keras, specifically targeting a scenario where image data is provided in a 3-dimensional format with channels as the first dimension. The fix ensures that the mean tensor used for normalization is correctly reshaped, preventing incorrect preprocessing and improving the robustness of image input pipelines for models expecting this specific data arrangement.

Highlights

• Bug Fix: Addressed a bug in the _preprocess_tensor_input function that occurred when the input x was 3-dimensional and the data_format was set to channels_first.
• Tensor Reshaping Logic: Modified the reshaping logic for mean_tensor to correctly handle 3D inputs with channels_first format, ensuring proper zero-centering.

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[gemini-code-assist]

Code Review

This pull request correctly fixes a bug in _preprocess_tensor_input that occurred when preprocessing a 3D tensor with data_format='channels_first'. The previous implementation produced an incorrect shape for the mean_tensor, leading to a broadcasting error. The fix introduces a special case for 3D tensors which resolves the issue. My review includes a minor suggestion to improve the readability of the new code by simplifying an expression, in line with the Keras API design guidelines on minimizing cognitive load.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.84%. Comparing base (a5e6d0a) to head (647d87d).

Files with missing lines	Patch %	Lines
keras/src/applications/imagenet_utils.py	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21750      +/-   ##
==========================================
+ Coverage   82.63%   84.84%   +2.21%     
==========================================
  Files         572      572              
  Lines       58551    58553       +2     
  Branches     9151     9152       +1     
==========================================
+ Hits        48381    49682    +1301     
+ Misses       7843     6428    -1415     
- Partials     2327     2443     +116     

Flag	Coverage Δ
keras	84.66% <33.33%> (+2.22%)	⬆️
keras-jax	65.43% <33.33%> (+2.24%)	⬆️
keras-numpy	57.67% <0.00%> (+0.10%)	⬆️
keras-openvino	34.50% <0.00%> (+0.12%)	⬆️
keras-tensorflow	66.19% <33.33%> (+2.24%)	⬆️
keras-torch	65.73% <33.33%> (+2.22%)	⬆️
keras.applications	83.52% <33.33%> (?)
keras.applications-jax	83.52% <33.33%> (?)
keras.applications-numpy	22.79% <0.00%> (?)
keras.applications-openvino	22.79% <0.00%> (?)
keras.applications-tensorflow	83.52% <33.33%> (?)
keras.applications-torch	83.25% <33.33%> (?)

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