add pv padding and hf model handling to backtest_sites.py

[AUdaltsova]

Pull Request

Description

• Added an option to use a model from HuggingFace
• Added datapipe to pad pv data so that sun position is created correctly (see this issue in dpipes)
• Fixed conversion to nanoseconds warning

How Has This Been Tested?

Ran small backtest sthat require pv padding with a local and a HF model

• [X] Yes

If your changes affect data processing, have you plotted any changes? i.e. have you done a quick sanity check?

Checked that with new coded padding the forecast is identical to what a manually padded dataset produces.

• [X] Yes

Checklist:

• [X] My code follows OCF's coding style guidelines
• [X] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works
• [X] I have checked my code and corrected any misspellings