Merge ArithmeticNode into ExecuteNode (#2247)

Summary:
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Pull Request resolved: https://github.com/pytorch/executorch/pull/2247

This diff moves the logic of `ArithmeticNode` into its corresponding OpFunction `add_arithmetic_node()` and the `ExecuteNode` class.

Our aim is to remove all derived classes of `ExecuteNode`, i.e., to make `ExecuteNode` a final class. All operator-specific logic will be handled in the OpFunction.

Note the next change will move `StagingNode` into its OpFunction + this new ExecuteNode implementation. Until then, we can't tidy up the `ExecuteNode` class fully. Finally, we leave a few task TODOs.
ghstack-source-id: 217439330
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Reviewed By: SS-JIA

Differential Revision: D53982441

fbshipit-source-id: b8a51eee538b679e4168864a4870f3921c9ba333