[RetinaNet]: General fixes

[ahmadki]

• Changed the default lr to something that works with ADAM
• Synced the the onnx conversion script with the latest model changes
• Cleaned up CocoEvaluator implementation

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[ahmadki]

@johntran-nv @emizan76

The PR is ready for review / merger. It doesn't change the benchmark but does include some quality of life improvements.