Add 'relational positional embedding (RPE)' feature to T5
# What does this PR do ?

Add 'relative positional embedding (RPE)' feature to T5 model.

**Collection**: NLP

# Changelog 
Adds an option to use 'relative positional embedding' other than 'absolute position embedding' or 'fixed position encoding'.

# Usage

In the 'megatron_t5_config.yaml' file change the 'position_embedding_type' option to 'relative'.

```python
python megatron_t5_pretraining.py
```

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- [ ] Did you write any new necessary tests?
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- [ ] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
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**PR Type**:
- [x] New Feature
- [ ] Bugfix
- [ ] Documentation

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# Additional Information
* Related to # (issue)