fix handling of the final word for rnnt word timestamps
# What does this PR do ?

Fixes another observed edge case for the final word in RNNT word timestamps

**Collection**: ASR

# Changelog 
- the last word in rnnt decoding for a segment may contain multiple tokens, but the data model for how these were aggregated 

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**Pre checks**:
- [X] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
  
**PR Type**:
- [ ] New Feature
- [X] Bugfix
- [ ] Documentation

# Additional Information
* Related to # (issue)

Example object from real use case
```python
from torch import tensor
offsets = [{'char': [tensor(456, dtype=torch.int32), tensor(1024, dtype=torch.int32)],
  'start_offset': 0,
  'end_offset': 1},
 {'char': [tensor(28, dtype=torch.int32),
   tensor(291, dtype=torch.int32),
   tensor(1024, dtype=torch.int32)],
  'start_offset': 2,
  'end_offset': 3}]
built_token = [456, 28, 291]
```

If you attempt to index offsets via `offsets[-len(built_token)]]` you get an error because `len(offsets)==2` and `len(built_token)==3`