Introduce base `Backend` #7472

Job	Run time
Build, test and upload wheel	1m 18s
Framework / bare	2m 48s
Framework / bare-https	1m 15s
Framework / jax	1m 48s
Strategy / FedMedian	1m 15s
Framework / pytorch	2m 41s
Strategy / FedTrimmedAvg	1m 19s
Framework / tensorflow	3m 17s
Strategy / QFedAvg	1m 29s
Framework / tabnet	2m 47s
Strategy / FaultTolerantFedAvg	1m 12s
Framework / opacus	2m 48s
Strategy / FedAvgM	1m 12s
Framework / pytorch-lightning	2m 38s
Strategy / FedAdam	1m 10s
Framework / scikit-learn	2m 40s
Strategy / FedAdagrad	1m 11s
Framework / fastai	4m 2s
Strategy / FedYogi	1m 14s
Framework / pandas	1m 42s
	39m 46s

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