Add cmdline args method to DialogPartnerWorld

[EricMichaelSmith]

Patch description
#3480 added an .add_cmdline_args() method to SelfChatWorld, given the recent work on mutators; this PR moves that method up one level to DialogPartnerWorld so that it can be used by any interactive world (such as the multi-dimensional gender classifier) or any self-chat world.

Testing steps

Testing interactive mode with multi-dimensional gender classifier, made working again by this PR:

parlai interactive \
-t md_gender \
-m projects.md_gender.bert_ranker_classifier.agents:BertRankerClassifierAgent \
-mf zoo:md_gender/model \
-ecands inline \
-cands inline \
--interactive_mode False \
--data-parallel False

Testing self-chat, made working by the #3480 but refactored here:

parlai self_chat \
--model-file zoo:blender/blender_90M/model \
--task blended_skill_talk \
--outfile ~/__test_self_chat.jsonl