Megatron KERPLE positional embeddings
# What does this PR do ?

This PR adds support to KERPLE (https://arxiv.org/abs/2205.09921) positional embeddings for Megatron encoder-decoder classes.

**Collection**: nlp megatron

# Changelog 

- Added implementation of KERPLE
- Added Jenkins test
- Added comment to document feature in YAML file (`examples/nlp/language_modeling/conf/megatron_model_base_config.yaml`)

# Usage

Set config
```
model.encoder.position_embedding_type: 'kerple'
model.decoder.position_embedding_type: 'kerple'
```


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

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# Additional Information
* Related to # (issue)
