bug fix about usage of absl::flat_hash_map::erase and absl::flat_hash_set::erase

[wumuzi520]

Why are these changes needed?

It is wrong to use absl::flat_hash_map::erase or absl::flat_hash_set::erase like this:

for (const auto &entry : flat_hash_set) {
   .....
   flat_hash_set.erase(entry);
}

for (auto iter = flat_hash_set.begin(); iter != flat_hash_set.end(); ++iter) {
   .....
   flat_hash_set.erase(iter);
}

The tow above codes will lead to exception.

Related issue number

Checks

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• I've included any doc changes needed for https://ray.readthedocs.io/en/latest/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failure rates at https://ray-travis-tracker.herokuapp.com/.
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested (please justify below)

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