Perplexity is computed incorrectly
**Describe the bug**

Perplexity is computed incorrectly [here](https://github.com/NVIDIA/NeMo/blob/main/nemo/collections/nlp/metrics/perplexity.py) and in [TransformerLMModel](https://github.com/NVIDIA/NeMo/blob/main/nemo/collections/nlp/models/language_modeling/transformer_lm_model.py). Perplexity is an exponent of entropy whereas in these scripts it is computed as an exponent of cross-entropy.
