amend rnnt word timestamps
# What does this PR do ?

This PR amends the start time for the last token in RNNT decoding because the code aggregates (inclusively) since the previous_token_index all tokens in the current word. This has tripped me up in my own implementations of similar code external to NeMo in the past. My apologies for not catching it in #4779. Without these changes, some transcription invocations fail.

**Collection**: ASR

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**PR Type**:
- [X] Bugfix

@titu1994 I tested this code against an ASR corpus of 163,708 words, and it's looking stable with no more errors thrown. My tests were in WER rather than time stamps. There's a jitter of about 0.1%pt WER on clean English and 0.5%pts on noisy English, which is similar to what I've seen before on repeated invocations of custom models, though perhaps a bit larger for noisy audio. I don't know where to enumerate more test cases in this codebase, but this logic is a bit difficult to exhaustively test. I've seen 200-300 millisecond drifts in word timestamps across different models, so I don't know how to formalize a test for these outputs more specifically.