Add StdScaler for time series Transformer model
Author: kashif
Pull request: yes

# What does this PR do?

- [x] Add `loc` and `scale` outputs from current scalers and use both as static real-valued covariates
- [ ] double check training/inference works as usual
- [ ] add the StdScaler


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- [ ] Did you write any new necessary tests?


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Comments:
_The documentation is not available anymore as the PR was closed or merged._

thanks! will add... moving this to draft for now as i have to check this before adding

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20270). All of your documentation changes will be reflected on that endpoint.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20270). All of your documentation changes will be reflected on that endpoint.

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