Converting the list to ndarray for speedup

[askender]

Patch description
Creating a tensor from a list of numpy.ndarrays is extremely slow.
converting the list to a single numpy.ndarray with numpy.array() before converting to a tensor.

Testing steps
re-run parlai train_model --model projects.blenderbot2.agents.blenderbot2:BlenderBot2FidAgent ***, and no UserWarning again.

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[stephenroller]

Thanks!

[stephenroller]

(Please just sign the CLA)

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[askender]

I think the Some checks were not successful
such as E FileNotFoundError: [Errno 2] No such file or directory: '/home/circleci/ParlAI/data/wizard_of_wikipedia/data.json'
is not related to my pull-request.

@stephenroller Thanks. I will do more useful pull-request. ParlAI is cool.

[stephenroller]

I'll rerun tests on Monday. It's slightly suspicious but the code looks good