Channel mapper implementation has error

[JohnMBrandt]

Many DETR models use group normalization within channel mapper (e.g. DINO, DETA, etc.).

The implementation of channel mapper in this repo defaults bias = True, which is not best practice since group normalization has its own bias term.

In contrast, the channel mapper implementation in MMDetection relies on mmcv.ConvModule, which turns bias to False if a norm layer is attached (https://mmcv.readthedocs.io/en/2.x/api/generated/mmcv.cnn.ConvModule.html).

Suggest either turning bias to False by default, or updating the model configs that use ChannelMapper with group norm to have bias=False

In practice I get ~0.3 - 0.6 AP improvement from this