[s2s] distillation apex breaks return_dict obj
This is a continuation of https://github.com/huggingface/transformers/pull/8612 for `distillation.py` - this PR is switching from `.property` to `["property"]`.

Unfortunately, the script itself doesn't seem to work under apex even after the fix - perhaps it never was. 

But it's probably still OK to merge, since it no longer fails with the #8530-related symptoms and is in sync with `finetune.py` now.

I filed a separate issue about it: https://github.com/huggingface/transformers/issues/8632

@sgugger 