[飞桨黑客松第四期](137)add MoCo-v2 and BEiT-v2 rfcs

[oozhuzaioo]

任务详情：PaddlePaddle/Paddle#51281 (comment)

新增文档

• rfc_task-137_model_MoCo-v2.md
• rfc_task-137_model_BEiT-v2.md

[paddle-bot]

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