🚀 The feature, motivation and pitch

This is part of #147479

Probably should divide it into 3 parts.

1. Remove references of autotune_fallback_to_aten in tests, benchmarks, etc. If autotune_fallback_to_aten is set to False, we can safely remove it.
2. Remove usage of autotune_fallback_to_aten inside inductor.
3. Remove env setting of autotune_fallback_to_aten in config.py, and say it is deprecated.

Alternatives

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Additional context

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cc @voznesenskym @penguinwu @EikanWang @jgong5 @Guobing-Chen @XiaobingSuper @zhuhaozhe @blzheng @wenzhe-nrv @jiayisunx @ipiszy @chenyang78 @kadeng @muchulee8 @amjames @chauhang @aakhundov