rnnt and char utils
# What does this PR do ?

fix ngram_merge using rnnt nemo model.
fix char level ngram creation.

**Collection**: ASR

# Changelog 
- scripts/asr_language_modeling/ngram_lm/ngram_merge.py count perplexity of a test set using tokenizer from RNNT model
- scripts/asr_language_modeling/ngram_lm/kenlm_utils.py create word-level ngram model using char-level nemo model

```python
# Add a code snippet demonstrating how to use this 
```

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