[WIP]: Add conditional attention for conditional detr

[Li-Qingyun]

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Motivation

I was chosen for the 1st GLCC mmdetection project in open-mmlab/mmdetection#8030, and I'm going to cooperate with @KeiChiTse add Conditional DETR to mmdetection. The attention modules of Conditional DETR decoder is different with MultiheadAttention in mmcv. The new attention module shoule be added into mmcv.cnn.bricks.transformer.

Main Modification

Modified mmcv/cnn/bricks/transformer.py to add implementations of ConditionalAttention

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[Li-Qingyun]

The work will be done by other contributor.