Add lru_cache for node comparison (#148)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/148

cache the args to optimize the number of calls. For `is_same_node` function, we're expecting the same results each time it's called with the same inputs.

For mv2,
```
buck run mode/dev-nosan  //executorch/examples/backend:xnnpack_examples -- -m mv2 -q -d
```
The total number of nodes in mv2 graph after quantize is 520.

Before the change:
mv2: The number of call to `is_same_node` is 6388036 🙈
mv2: to_backend call time: 30.79 second (including dumping the logs)
mv3: takes forever

After the change:
mv2: The number of call to `is_same_node` is 520. It's more reasonable
mv2: to_backend call time: 10.15 second (including dumping the logs)
mv3: `to_backend` call takes 16 second.

Reviewed By: digantdesai, mcr229

Differential Revision: D48708658

fbshipit-source-id: 8585c417d6d7c27bbacda563c0a7a7a764f0674e