add input_output_map and multi-vote

[zhaochenyang20]

Description

This one is decoupled from #196.

1. I add construct_input_output_map and apply_multi_vote_to_construct_generated_dataset with corresponding unit tests in dataset_generator_with_filter_test.py.
2. I change mock_batch_openai_response to mock_batch_openai_response_with_identical_completions. Because later, we need new mock functions called mock_batch_openai_response_with_different_completions and reset_mock_batch_openai_response_with_different_completions.
3. I add dataset_tool in test_helper, which consists of are_datasets_identical and are_dataset_dicts_identical. I also use these two functions in the unit tests of dataset_processor in the final PR.

References

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Blocked by

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[neubig]

I approve, with one small suggested change!