Update data_prep.py
When batch_size >= 2, the align.py doesn't work on the batch_size examples. This function should return [start, end] (inclusive) examples.

# What does this PR do ?

Fix a function logic:  https://github.com/NVIDIA/NeMo/blob/main/tools/nemo_forced_aligner/utils/data_prep.py#L78

**Collection**: [Note which collection this PR will affect]
nemo neural aligner
no collections will be affected

# Changelog 
- 

# Usage
* You can potentially add a usage example below

```python
# Add a code snippet demonstrating how to use this 
python NeMo/tools/nemo_forced_aligner/align.py \
         pretrained_name="stt_en_citrinet_1024_gamma_0_25" \
       model_downsample_factor=8 \
       manifest_filepath=./test_manifest.json \
      output_dir=./ctm_test_manifest
      batch_size=2
```

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  - [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation

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# Additional Information
* Related to # (issue)
