UL2 Mixture-of-Denoiser loss
### Feature request

The losses applied to the paper **UL2: Unifying Language Learning Paradigms**
The Mixture-of-Denoisers losses are described in the UL2 paper, which can be found at the following link: https://arxiv.org/abs/2205.05131
The code is based on T5x (which is JAX/FLAX): https://github.com/google-research/t5x


### Motivation

I am requesting the addition of new losses applied in the UL2 paper called Mixture-of-Denoisers. These new losses have been shown to improve the performance of unsupervised learning models and I believe they could benefit the HuggingFace community.

### Your contribution

Opening the request
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