Execute on exec_aten namespace deprecation from #5296 (#7950)

fastmod 'exec_aten::' 'executorch::aten::'
Manually fix op_sdpa.cpp to use ::executorch::aten, run lintrunner.

(FWIW, I disagree with our use of nested namespaces for the reasons outlined in https://abseil.io/tips/130, but we can always find/replace back and leaving the migration unfinished seems worse.)