Add dp secagg demo example #8460

Job	Run time
Build, test and upload wheel	1m 16s
Strategy / FedMedian	1m 16s
Framework / bare	3m 1s
Strategy / FedTrimmedAvg	1m 12s
Framework / bare-https	1m 41s
Strategy / QFedAvg	1m 21s
Strategy / FaultTolerantFedAvg	1m 13s
Framework / jax	1m 56s
Strategy / FedAvgM	1m 11s
Framework / pytorch	2m 41s
Framework / tensorflow	3m 11s
Strategy / FedAdam	1m 35s
Strategy / FedAdagrad	1m 56s
Framework / tabnet	2m 45s
Strategy / FedYogi	1m 22s
Framework / opacus	2m 49s
Framework / pytorch-lightning	2m 43s
Framework / scikit-learn	2m 56s
Framework / fastai	3m 55s
Framework / pandas	1m 51s
	41m 51s

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