[Executorch][BE] Rename sdpa_with_kv_cache.py to custom_ops.py (#7210)

Pull Request resolved: https://github.com/pytorch/executorch/pull/6996

Because now we have more than sdpa_with_kv_cache in it
ghstack-source-id: 256711931
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//oss complaining of internal lint failure, unit-test-arm broken in trunk
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Differential Revision: [D66269486](https://our.internmc.facebook.com/intern/diff/D66269486/)

Co-authored-by: Kimish Patel <kimishpatel@fb.com>