[WIP] Add conditional detr

[Li-Qingyun]

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Motivation

I was chosen for the 1st GLCC project in #8030, and I'm going to cooperate with @KeiChiTse add Conditional DETR to mmdetection.

Main Modification

• Create conditional_detr.py to implement ConditionalDETR
• Create conditional_detr_head.py to implement ConditionalDETRHead
• Modified transformer.py to add implementations of ConditionalDetrTransformerDecoderLayer and ConditionalDetrTransformerDecoder
• The attention module should be rewritten, hence, open-mmlab/mmcv#2114 is created for implementing ConditionalAttention.

[CLAassistant]

All committers have signed the CLA.

[Li-Qingyun]

The work will be done by other contributors.