Backbone add out indices
Author: amyeroberts
Pull request: yes

# 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.

## Before submitting
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- [ ] Did you write any new necessary tests?


## Who can review?

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