Remove max_prompt_length from experimental PRM

[albertvillanova]

Remove max_prompt_length from experimental PRM.

This PR removes support for the max_prompt_length parameter from the PRM trainer code and its associated tests.

Follow-up to:

• Refactor KTO coordinated with DPO [b/N]: Simplify truncation logic #4808
• Deprecate max_prompt_length in RLOOTrainer #4703
• Refactor DPO #3906

Key changes include:

Core code and configuration cleanup:

• Removed the max_prompt_length parameter and its logic from the PRMConfig class, the PRMTrainer.tokenize_row method, and related configuration and function signatures.
• Updated the truncation logic in tokenize_row removing prompt-specific truncation.

Testing updates:

• Removed all uses of max_prompt_length from test cases and deleted the test for prompt truncation, as this functionality is no longer supported.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[qgallouedec]

thanks