Retire custom AdamWithDecay

We have a custom Adam optimizer that says it could be removed via pytorch 1.2:

https://github.com/facebookresearch/ParlAI/blob/3a5de86667816abbdb0f08049c86d1e2c3aff03e/parlai/agents/bert_ranker/helpers.py#L242-L350

The task is to remove it and upgrade to the official pytorch version.

The helper function minimizes the pain required, though I'm wondering if we'll need to upgrade some state dicts, so make sure you run long tests. https://github.com/facebookresearch/ParlAI/blob/3a5de86667816abbdb0f08049c86d1e2c3aff03e/parlai/agents/bert_ranker/helpers.py#L205