Revise selected unit-tests for inference

[sidgoyal78]

For the inference api, the current c++ unit tests for sequence based book chapters, don't handle the decoding part correctly (this is partly because the decode ops are currently being worked on).

So this issue is just to track the unit-tests that need modification, the chapters and relevant PRs are:

• semantic role labelling (PR Inference example and unit test for label_semantic_roles #8058)
• rnn encoder decoder (PR Add Inference example and unit test for rnn_encoder_decoder #8176)
• NMT (not merged till now: PR Inference example and inference-framework unit-test for NMT model #8314)