Perform evaluation on HANS with Trainer (like GLUE example)
Current [HANS](https://github.com/huggingface/transformers/tree/master/examples/adversarial) evaluation implementation is carried out in the old way. It'd be good to do it in the same manner as other examples are implemented now with the Trainer class. 

Maybe @sgugger would be interested in taking a look at this<|||||>There is an evaluation issue that I faced with HANS evaluation. Issue [here](https://github.com/huggingface/transformers/issues/4766). I tried to run in the exact same manner as listed in `examples/adversarial`. But the results obtained seem skewed for some reason. If @sgugger can point some possible reason out, I can work on it and send a PR. Thanks.