[BUG] Use get_cls in _BasePtForecaster_Common helper methods

[Ajeem-git]

Summary: Fix dormant AttributeError paths in _BasePtForecaster_Common by replacing calls to the undefined get_model_cls() with the existing get_cls() helper.

What changed:

• Updated name() fallback to use get_cls().
• Updated create_test_instance() to instantiate via get_cls().
• Updated create_test_instances_and_names() to instantiate via get_cls().

Why:
_BasePtForecaster_Common defines get_cls(), but three helper paths were calling get_model_cls(), which does not exist. This leaves dormant AttributeError paths in the base helper methods.

Verification:

• Verified the modified module compiles with python3.11 -m py_compile pytorch_forecasting/models/base/_base_object.py.
• Confirmed the issue reproduction matches the code path described in [BUG] get_model_cls() called but never defined in _BasePtForecaster_Common #2187.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@af9b14f). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2244   +/-   ##
=======================================
  Coverage        ?   86.94%           
=======================================
  Files           ?      167           
  Lines           ?     9782           
  Branches        ?        0           
=======================================
  Hits            ?     8505           
  Misses          ?     1277           
  Partials        ?        0           

Flag	Coverage Δ
cpu	86.94% <100.00%> (?)
pytest	86.94% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.