Remove torchvision dep and simplify resize and rgb_to_grayscale in torch backend

[james77777778]

We can simplify the logic of resize and rgb_to_grayscale and remove the dep of torchvision

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.00000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 82.24%. Comparing base (c04cf9d) to head (4ee27b6).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
keras/src/backend/torch/image.py	90.00%	2 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #20868   +/-   ##
=======================================
  Coverage   82.24%   82.24%           
=======================================
  Files         561      561           
  Lines       52633    52642    +9     
  Branches     8137     8134    -3     
=======================================
+ Hits        43288    43297    +9     
- Misses       7340     7341    +1     
+ Partials     2005     2004    -1     

Flag	Coverage Δ
keras	82.05% <90.00%> (+<0.01%)	⬆️
keras-jax	64.21% <8.00%> (-0.01%)	⬇️
keras-numpy	59.02% <8.00%> (-0.01%)	⬇️
keras-openvino	32.51% <0.00%> (-0.01%)	⬇️
keras-tensorflow	64.84% <8.00%> (-0.01%)	⬇️
keras-torch	64.28% <90.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[fchollet]

Very nice, thank you!