Fix: Pass attn_implementation override to CriticWorker

[arde171]

What does this PR do?

Problem

CriticWorker was extracting `attn_implementation` from `override_config` but not passing it to `load_valuehead_model()`, causing models to always default to `flash_attention_2` regardless of configuration.

Solution

• Added `attn_implementation` parameter to `load_valuehead_model()` function
• Implemented smart fallback: explicit param → model_config._attn_implementation → 'flash_attention_2' default
• Updated CriticWorker to pass `attn_implementation` to `load_valuehead_model()`

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