fix onmt converter #2600

Job	Run time
check-python-style	8s
build-and-test-cpp-aarch64 (openblas)	4m 17s
build-and-test-cpp-x86_64 (ubuntu-20.04, mkl)	6m 10s
build-python-wheels (ubuntu-20.04, auto64)	27m 18s
build-and-push-docker-images	23m 15s
build-and-test-cpp-x86_64 (ubuntu-20.04, dnnl)	5m 27s
build-python-wheels (macos-11, auto64)	35m 1s
build-and-test-cpp-x86_64 (macos-11, mkl)	11m 41s
build-and-test-cpp-x86_64 (macos-11, dnnl)	9m 4s
build-python-wheels (windows-2019, auto64)	42m 33s
build-python-wheels (ubuntu-20.04, aarch64)	1h 12m 36s
build-python-wheels (macos-11, arm64)	10m 59s
build-and-deploy-docs	42s
test-python-wheels (ubuntu-20.04)	8m 43s
test-python-wheels (macos-11)	5m 51s
test-python-wheels (windows-2019)	4m 2s
publish-python-wheels-on-pypi	0s
	4h 27m 47s

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