max_qa_length is needed for funetune on multiple-choice problems
# 🚀 Feature request
In examples/multiple-choice/run_multiple_choice.py, the max_qa_length is not implemented as that in google/Albert, which would cause trouble when funetuning RACE.
This should be easy becauce there are similar codes in codes for run_squad.py.
### Your contribution
I have done the script and get similar RACE performance compared to google/Albert.

# Current Bug
In examples/multiple-choice/utils_multiple_choice.py
```python
# From 537 lines: 
            # It should be => 
            # inputs = tokenizer.encode_plus(
            #    text_a, text_b, add_special_tokens=True, max_length=max_length, pad_to_max_length=True, return_overflowing_tokens=True
            # )
            # Or the log info below would never be activated.
            inputs = tokenizer.encode_plus(
                text_a, text_b, add_special_tokens=True, max_length=max_length, pad_to_max_length=True,
            )
            if "num_truncated_tokens" in inputs and inputs["num_truncated_tokens"] > 0:
                logger.info(
                    "Attention! you are cropping tokens (swag task is ok). "
                    "If you are training ARC and RACE and you are poping question + options,"
                    "you need to try to use a bigger max seq length!"
                )
```

Indeed, you're totally correct. Thank you for reporting the issue, I'm fixing it in #4385.