Fix default tokenizer for HuggingFace Agents

[Rebecca-Qian]

Patch description
Updates the HuggingFace agent to over-ride is_prebuilt to return True, since HuggingFace agents build their own dictionaries.

Using the default tokenizer setting with T5 caused the exception ValueError('Dictionaries should be pre-built before distributed train.')

Testing steps

parlai mp_train -m parlai.agents.hugging_face.t5:T5Agent -mf /tmp/model_file -t jsonfile --jsonfile-datapath $PATH --fp16 true --t5-model-parallel False --ddp-backend zero2 --jsonfile-datatype-extension True

[klshuster]

thank you for this!!

[stephenroller]

yay