Move chat templates from inline strings to .jinja files

[qgallouedec]

Moves the four inline Jinja2 chat template strings from chat_template_utils.py into separate .jinja files under trl/chat_templates/, with a README explaining their purpose and the training template diffs.

These templates are large (up to ~8K chars) and make chat_template_utils.py hard to navigate. Storing them as .jinja files makes them easier to read, diff, and edit and enables syntax highlighting in editors.

Requires #5458

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Note

Medium Risk
Moderate risk because trl.chat_template_utils now reads template text from package files at import time; missing/incorrectly packaged template files would break schema detection and training template patching for Qwen models.

Overview
Moves the Qwen3/Qwen3.5 Jinja2 chat templates out of chat_template_utils.py into standalone files under trl/chat_templates/, and updates the Python constants to load them via Path(...).read_text().

Adds trl/chat_templates/README.md documenting the identity-comparison vs training-patch templates, and updates MANIFEST.in to include the new .jinja and .md template assets in source distributions.

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[albertvillanova]

Thanks.