[MODEL] fix seq2seq encoder input order

[szha]

Description

fixes #1367

Checklist

Essentials

• [x] PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• [x] Changes are complete (i.e. I finished coding on this PR)
• [x] All changes have test coverage
• [x] Code is well-documented

Changes

• [x] fix seq2seq encoder input order

cc @dmlc/gluon-nlp-team

[mli]

Job PR-1368/1 is complete. Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-1368/1/index.html

[szha]

@sxjscience thanks for the quick review!