fix output_dir / tokenizer_name confusion
Fixing #3950 

It seems that `output_dir` has been used in place of `tokenizer_name` in `run_xnli.py`. I have corrected this typo.

I am not that familiar with code style in this repo, in a lot of instances I used:

`args.tokenizer_name if args.tokenizer_name else args.model_name_or_path`

Instead, I believe I could have set `args.tokenizer_name = args.tokenizer_name if args.tokenizer_name else args.model_name_or_path` somewhere at the start of the script, so that the code could be cleaner below.
No, this isn't correct. This was consistent with other scripts (before they were ported to the new Trainer in #3800) where we always saved the model and its tokenizer before re-loading it for eval 

So I think that code is currently right. (but again, should be re-written to Trainer pretty soon)