[Fix]: GTN, fastGTN, MHNF model 'RuntimeError' when using gpu

Description

Fix the 'RuntimeError: indices should be either on cpu or on the same device as the indexed tensor (cpu)' problem for [GTN] [fastGTN] [MHNF] models.

Please feel free to remove inapplicable items for your PR.

[x] The PR title starts with [$CATEGORY] (such as [NN], [Model], [Doc], [Feature]])
[x] Changes are complete (i.e. I finished coding on this PR)
[x] All changes have test coverage
[ ] Code is well-documented
[x] To the best of my knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change
[ ] Related issue is referred in this PR
[ ] If the PR is for a new model/paper, I've updated the example index here.