[ENH] Add v2 support for DecoderMLP - closes #2076

[BLVCK-MAMBA-6]

Summary

Closes #2076

Implements DecoderMLP for the v2 pipeline by inheriting from TslibBaseModel,
following the pattern established by DLinear in _dlinear_v2.py.

Changes

• pytorch_forecasting/models/mlp/_decodermlp_v2.py — new v2 implementation
• pytorch_forecasting/models/mlp/__init__.py — exports DecoderMLPV2
• tests/test_models/test_decodermlp_v2.py — 5 tests, 85% coverage

Test results

All 5 tests passing locally:

• test_decodermlp_init ✅
• test_decodermlp_forward ✅
• test_decodermlp_quantile_loss ✅
• test_decodermlp_no_nan ✅
• test_decodermlp_univariate ✅

This is part of my GSoC 2026 application for Project 2.2 (v1→v2 model migration).

[codecov]

Codecov Report

❌ Patch coverage is 85.18519% with 8 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@5600398). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
pytorch_forecasting/models/mlp/_decodermlp_v2.py	84.61%	8 Missing ⚠️

Additional details and impacted files

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  Coverage        ?   86.61%           
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  Files           ?      166           
  Lines           ?     9789           
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  Hits            ?     8479           
  Misses          ?     1310           
  Partials        ?        0           

Flag	Coverage Δ
cpu	86.61% <85.18%> (?)
pytest	86.61% <85.18%> (?)

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