Move hard-coded dropout and staleness values into configs

[dfulu]

Pull Request

Description

Previously the dropout and max staleness values used in the pvnet and windnet datapipes were hard-coded into their common functions. This moves these values to the config file.

Also a little tidying

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: 7 lines in your changes are missing coverage. Please review.

Comparison is base (e45f900) 78.20% compared to head (aea4645) 78.33%. Report is 1 commits behind head on main.

:exclamation: Current head aea4645 differs from pull request most recent head d398490. Consider uploading reports for the commit d398490 to get more accurate results

Files	Patch %	Lines
ocf_datapipes/training/common.py	82.60%	4 Missing :warning:
ocf_datapipes/config/model.py	93.18%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #265 +/- ## ========================================== + Coverage 78.20% 78.33% +0.13% ========================================== Files 121 121 Lines 5551 5595 +44 ========================================== + Hits 4341 4383 +42 - Misses 1210 1212 +2 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.