Multi-pretrained weights: Add initial API and basic implementation

[datumbox]

🚀 The feature

Part of #4611

Add the initial API mechanism for the Multi-pretrained weights:

• The WeightEntry and Weights classes.
• The necessary preset transforms.
• The meta information needed for the models.

Moreover provide basic implementations for the following models:

• ResNet50
• FasterRCNN + ResNet50
• Quantized ResNet50