Add PVnet weighted loss

[jacobbieker]

Pull Request

Description

Adds weighted loss for quantile loss to help with longer forecast horizons, and ensuring the model focuses on shorter timescales as well.

Fixes #153

How Has This Been Tested?

Unit tests

• [x] Yes

Checklist:

• [x] My code follows OCF's coding style guidelines
• [x] I have performed a self-review of my own code
• [x] I have made corresponding changes to the documentation
• [x] 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

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 61.43%. Comparing base (fe5fd2f) to head (1d5a859). Report is 1 commits behind head on main.

Files	Patch %	Lines
pvnet/models/base_model.py	33.33%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #165 +/- ## ========================================== - Coverage 61.47% 61.43% -0.05% ========================================== Files 28 28 Lines 1786 1789 +3 ========================================== + Hits 1098 1099 +1 - Misses 688 690 +2 ```

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