Add gpu option to torchscript BART models

[maryamdaneshi]

Patch description
Parsing no_cuda to enable/disable scripting for gpu. additionally, make the changes needed to script for gpu

Testing steps

export gpu model

parlai torchscript \
--model-file ${MODEL_FILE} \
--model bart \
--no-cuda false \
--scripted-model-file ~/_test_scripted_model__bart.pt \
--input 'I am looking for a restaurant in the west part of town.|APIRESP: Restaurant 14 matches'

Interact with the model

parlai interactive \
--model-file ${SCRIPTED_MODEL_FILE} \
--model bart \
--no-cuda false

output

Generating given the original unscripted module and the scripted module is the same, unit tests to prove that 

[EricMichaelSmith]

Makes sense, thanks! Minor comment

[EricMichaelSmith]

Hmm instead of adding the --script-for-gpu flag, could we just reuse the value of the no_cuda key?

[stephenroller]

We should add a GPU test