FIx PytorchObjectDetector Loss Gradient Bug

[f4str]

Description

Add the missing if-condition when setting gradients in the PyTorchObjectDetector estimator such that loss gradients can be properly calculated. This if-condition is copied from PyTorchYolo.

Fixes #2237

Type of change

Please check all relevant options.

• Improvement (non-breaking)
• Bug fix (non-breaking)
• New feature (non-breaking)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Testing

Please describe the tests that you ran to verify your changes. Consider listing any relevant details of your test configuration.

• Tests for PyTorchObjectDetector are unchanged
• Tests for PyTorchFasterRCNN are unchanged

Test Configuration:

• OS
• Python version
• ART version or commit number
• TensorFlow / Keras / PyTorch / MXNet version

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[codecov-commenter]

Codecov Report

Merging #2238 (37628a6) into dev_1.15.1 (c7f0a4a) will decrease coverage by 0.01%.
The diff coverage is 33.33%.

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@@              Coverage Diff               @@
##           dev_1.15.1    #2238      +/-   ##
==============================================
- Coverage       85.62%   85.61%   -0.01%     
==============================================
  Files             308      308              
  Lines           27448    27450       +2     
  Branches         5044     5045       +1     
==============================================
  Hits            23502    23502              
- Misses           2667     2668       +1     
- Partials         1279     1280       +1     

Files Changed	Coverage Δ
...mators/object_detection/pytorch_object_detector.py	85.54% <33.33%> (-1.01%)	⬇️

[beat-buesser]

Hi @f4str Thank you very much for fixing this bug! The changes look good to me.