[NLP] Support T5 with Megatron Core
# What does this PR do ?

Adds megatron core support for our T5 model. This only works for pre-training, due to dynamic max lengths in the GLUE/XLNI datasets fine-tuning seems to be broken. 

The fix going forward I think will be to pad to the maximum length to ensure the size of sequences are always the same. This is a requirement of the iterator object now used in the `training_step`.

I also need to confirm that the weights of the model are the same between main and this branch.

- [ ] Fix support for GLUE/XLNI Fine-tuning
- [x] Ensure convergence is the same
- [x] Fix support for encode/decode functions

**Collection**: NLP

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

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# Additional Information
* Related to # (issue)
