Wrong refactoring of mandatory parameters for run_squad.py
When only running evaluation on a squad dev set, it should *not* be mandatory to add a --train_file because only the --predict_file is necessary.

Current script invocation:
```
python run_squad \
    --model_type bert \
    --model_name_or_path xxx \
    --output_dir xxx \
    --train_file UNNECESSARY_BUT_MANDATORY \
    --predict_file xxx \
    --version_2_with_negative \
    --do_eval \
    --per_gpu_eval_batch_size 2
```

Desired script invocation without --train_file param:
```
python run_squad \
    --model_type bert \
    --model_name_or_path xxx \
    --output_dir xxx \
    --predict_file xxx \
    --version_2_with_negative \
    --do_eval \
    --per_gpu_eval_batch_size 2
```

Before refactoring in 50b7e52 the behavior was correct.
indeed, we could remove this<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
