update get_forecast_distance to handle gap

[fjlanasa]

Pull Request Description

Resolves #4201

This updates get_forecast_period to account for the gap parameter, so we only show predictions for X[-1] + gap to forecast_horizon.

---

After creating the pull request: in order to pass the release_notes_updated check you will need to update the "Future Release" section of docs/source/release_notes.rst to include this pull request by adding :pr:123.

[codecov]

Codecov Report

Merging #4200 (8455d93) into main (af53c15) will not change coverage.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main   #4200   +/-   ##
=====================================
  Coverage   99.7%   99.7%           
=====================================
  Files        349     349           
  Lines      38229   38229           
=====================================
  Hits       38111   38111           
  Misses       118     118           

Impacted Files	Coverage Δ
...valml/pipelines/time_series_regression_pipeline.py	100.0% <ø> (ø)
...peline_tests/test_time_series_baseline_pipeline.py	100.0% <100.0%> (ø)

[jeremyliweishih]

LGTM

[jeremyliweishih]

LGTM