adding special_tokens from tokenizer config for transformer-lm model
# What does this PR do ?

`special_tokens` are currently ignored when constructing a tokenizer for `transformer_lm_model` using the following example: https://github.com/NVIDIA/NeMo/blob/5cb76a53f21d7470773a80d62fd513defa24ddbf/examples/nlp/language_modeling/conf/bert_pretraining_from_text_config.yaml#L51

**Collection**: NLP/language-modeling

# Changelog 
- transformer_lm_model: use `special_tokens` from config when creating the tokenizer, making it backward-compatible with previous behavior (when `special_tokens` are not provided we proceed with `None`).

# Usage
* https://github.com/NVIDIA/NeMo/blob/5cb76a53f21d7470773a80d62fd513defa24ddbf/examples/nlp/language_modeling/conf/bert_pretraining_from_text_config.yaml#L51

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**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation

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# Additional Information
* Related to # (issue)
