Add MegatronT5
### Model description

In NeMo Megatron, the T5 model is available, but there is currently no MegatronT5 class for huggingface, such as MegatronBERT or MegatronGPT2. I have recently finished the porting work and have tested the model internally. I would like to share this model with the community.



### Open source status

- [X] The model implementation is available
- [ ] The model weights are available

### Provide useful links for the implementation

- [NeMo Megatron models](https://docs.nvidia.com/deeplearning/nemo/user-guide/docs/en/stable/nlp/nemo_megatron/intro.html)
- [NeMo](https://github.com/NVIDIA/NeMo)
- [Megatron-LM T5 model](https://github.com/NVIDIA/Megatron-LM/blob/main/megatron/model/t5_model.py)