Implements the UL2 Dataset and config
# What does this PR do ?

1. Implements the UL2 Dataset from https://arxiv.org/abs/2205.05131.
2. Adds a `megatron_ul2_config.yaml` to train an encoder-decoder model on this data using t5 pretraining script.

**Collection**: NLP

# Changelog 
- UL2 dataset class that inherits from T5Dataset
- UL2 yaml config
- Minor refactor of LMAdaptedT5Dataset

# Usage

`python megatron_t5_pretraining.py -cn megatron_ul2_config`

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

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# Additional Information
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