Fix "same" padding torch issue

[sachinprasadhs]

This fixes the issue for all torch layers when used with padding="same" and data_format="channels_first".

This was not caught in any of the test case due to the reason that it only happens with the above condition and when the padding_size is obtained from _compute_padding_length is something like ((0,0), (1, 1))

Fixes: #20235

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 73.41%. Comparing base (7628a18) to head (b5e23e2).
Report is 21 commits behind head on master.

Files with missing lines	Patch %	Lines
keras/src/backend/torch/nn.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #20270      +/-   ##
==========================================
+ Coverage   66.34%   73.41%   +7.06%     
==========================================
  Files         508      511       +3     
  Lines       48264    48737     +473     
  Branches     8882     8983     +101     
==========================================
+ Hits        32020    35778    +3758     
+ Misses      14425    11155    -3270     
+ Partials     1819     1804      -15     

Flag	Coverage Δ
keras	73.34% <0.00%> (+6.99%)	⬆️
keras-jax	62.43% <0.00%> (?)
keras-numpy	57.58% <0.00%> (?)
keras-tensorflow	63.74% <0.00%> (-2.60%)	⬇️
keras.applications	?
keras.applications-jax	?
keras.applications-numpy	?
keras.applications-tensorflow	?
keras.applications-torch	?

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[fchollet]

Thanks, Sachin. It looks like relevant torch tests are failing, can you take a look? https://github.com/keras-team/keras/actions/runs/10931265787/job/30345878233?pr=20270