feat(scripts): update torchvision syntax and add performance benchmark notebook

[frgfm]

This PR:

• updates all uses of torchvision models and transforms to the latest recommended syntax
• skip NaNs in metrics evaluation
• ensure CAM cannot run on models with multiple FC layers
• clean up documentation and add performance benchmark notebook

[codecov]

Codecov Report

❌ Patch coverage is 88.23529% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.35%. Comparing base (b13f9d4) to head (b1e9034).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
torchcam/methods/activation.py	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #367      +/-   ##
==========================================
- Coverage   97.56%   97.35%   -0.21%     
==========================================
  Files           8        8              
  Lines         452      454       +2     
==========================================
+ Hits          441      442       +1     
- Misses         11       12       +1     

Files with missing lines	Coverage Δ
torchcam/methods/_utils.py	100.00% <100.00%> (ø)
torchcam/methods/gradient.py	100.00% <ø> (ø)
torchcam/metrics.py	98.00% <100.00%> (+0.32%)	⬆️
torchcam/methods/activation.py	96.96% <71.42%> (-0.73%)	⬇️

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