What does this PR do?

Add out_indices as a way to specify which feature maps are returned in the backbone.

This isn't strictly necessary for cross loading timm backbones and is an optional design choice.

Reasoning:

• Greater compatibility between timm and transformer models when loading with AutoBackbone. For the same model e.g. microsoft/resnet-50 and resnet50, the stage names are different, whereas the layer index is the same. out_indices=(1,) means the same for both model. whereas selecting features requires knowing one uses layer1 and the other uses stage1.
• out_features requires knowing the names of the layers in order select which layers you want and is more prone to errors with typos
• out_indices is the param used in timm. Once advantage of out_indices is that you can do negative indexing easily. For example, to get the last two feature maps, you need only pass out_indices=(-2, -1).

Outstanding question on whether both out_features and out_indices should exist at the same time.

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