Add gradient visualization during validation

[sokovninn]

This pull request introduces a new callback for visualizing gradients using Grad-CAM during validation in the luxonis_train package. The most important changes include the addition of the GradCamCallback class, updates to the callback registration, and documentation updates.

New Callback Implementation:

• luxonis_train/callbacks/gradcam_visializer.py: Added the GradCamCallback class and its supporting ModelWrapper class to enable gradient visualization using Grad-CAM during validation.

Callback Registration:

• luxonis_train/callbacks/__init__.py: Registered the GradCamCallback class to make it available for use in the training pipeline.

Documentation:

• luxonis_train/callbacks/README.md: Updated the documentation to include a description of the GradCamCallback and its parameters.

[klemen1999]

Overall LGTM, left some comments

[kozlov721]

LGTM

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 42 lines in your changes missing coverage. Please review.

Project coverage is 96.31%. Comparing base (6146da6) to head (3de20bf).
Report is 8 commits behind head on main.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
luxonis_train/callbacks/gradcam_visializer.py	31.14%	42 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #115      +/-   ##
==========================================
- Coverage   97.12%   96.31%   -0.81%     
==========================================
  Files         139      147       +8     
  Lines        6012     6304     +292     
==========================================
+ Hits         5839     6072     +233     
- Misses        173      232      +59     

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