Fix k2 build topo helper

[artbataev]

What does this PR do ?

Fix build_topo function for k2-based topologies.
This solution fixes failing tests tests/collections/asr/k2/test_ctc.py::TestCTCLossK2::test_case_small_blank_last + tests/collections/asr/k2/test_rnnt.py::TestRNNTLossK2::test_case_small_blank_last

Collection: [ASR]

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[GNroy]

LGTM, thanks!