* feat: support excess_length_strategy for RL trainers
Previously, RL data loading always dropped sequences exceeding
sequence_len. This adds support for the existing `excess_length_strategy`
config option (`drop`, `truncate`, `raise`) in RL training pipelines,
matching the behavior already available for SFT.
- `drop` (default): unchanged behavior, filters out long samples
- `truncate`: tokenizes text components, truncates responses to fit
within sequence_len while preserving the full prompt, then decodes
back to text. Handles DPO/IPO/ORPO/SIMPO and KTO datasets.
- `raise`: raises ValueError if any sample exceeds sequence_len
Closes#3547
* improve RL truncation strategy robustness and performance
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Co-authored-by: yurekami <yurekami@users.noreply.github.com>
Co-authored-by: Wing Lian <wing@axolotl.ai>
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