fix: Fixed IS-CAM weight computation

[frgfm]

This PR introduces the following modifications:

• fixes the weight computation of IS-CAM (typo introduced in feat: Added support for multiple target layers in CAM computation #89)
• fixes the warning message for FC layer resolution in base CAM

[codecov]

Codecov Report

Merging #92 (ef6dad1) into master (1a8448f) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #92      +/-   ##
==========================================
+ Coverage   96.30%   96.31%   +0.01%     
==========================================
  Files           7        7              
  Lines         325      326       +1     
==========================================
+ Hits          313      314       +1     
  Misses         12       12              

Impacted Files	Coverage Δ
torchcam/cams/cam.py	96.87% <100.00%> (+0.03%)	⬆️