Fixed T5 and GPT2 related bugs in the hugging_face agents

[michaelsaxon]

Patch description
Previously T5 dictionary was undefined and elements of the huggingface dictionary agent were uninitialized leading to immediate exceptions in the train step for T5 and at the beginning of the validation step for GPT2 when training using train_model.py and the latest versions of HuggingFace

Testing steps
Verify that python parlai/scripts/train_model.py --m hugging_face/t5 --t multiwoz_v22 --mf ~/t5test --truncate 512 --vstep 150 --max-train-steps 300 and python parlai/scripts/train_model.py --m hugging_face/gpt2 --t multiwoz_v22 --mf ~/gpt2test --truncate 512 --vstep 150 --max-train-steps 300 successfully terminate without exceptions

Other information
As discussed with Kurt and Andrew previously

[klshuster]

Thank you for the fix!

I think this all looks great. However, prior to approving, I do think that your original error actually uncovered the real issue; namely, that we don't define an unk token for GPT2. Could you please additionally modify the Gpt2DictionaryAgent to define self.unk_token?

Presumably you'd just override some of the functionality here

Looking at huggingface, i think we can just set it to <|endoftext|> like the other special tokens.

[github-actions]

This PR has not had activity in 30 days. Closing due to staleness.

[klshuster]

Is there any update here?

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[klshuster]

closing in favor of #5016