Fixed GPU-based scoring by copying to cpu tensor first before converting to numpy.

[x] Changes are complete (if posting work-in-progress code, prefix your pull request title with '[WIP]' until you can check this box.
[x] Unit tests pass (pytest)
[x] Were system tests modified? If so did you run these at least 5 times to account for the variation across runs?
[x] System tests pass (pytest test/system)
[x] Passed code style checking (./style-check.sh)
[x] You have considered writing a test
[x] Updated major/minor version in sockeye/__init__.py. Major version bump if this is a backwards incompatible change.
[x] Updated CHANGELOG.md

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awslabs / sockeye