Add tokenizer_config support to LoRA YAML configuration

[breitburg]

This PR adds support for specifying custom tokenizer configuration, including custom chat templates, directly in LoRA YAML config files.

Changes

• Add tokenizer_config to CONFIG_DEFAULTS with trust_remote_code default
• Support chat_template_file to load Jinja2 templates from file paths
• Support inline chat_template strings
• Update documentation with usage examples
• Add comprehensive unit tests

Usage

Users can now specify tokenizer configuration in their YAML config:

# Option 1: Load chat template from file
tokenizer_config:
  trust_remote_code: true
  chat_template_file: "path/to/chat_template.jinja"

# Option 2: Inline chat template string
tokenizer_config:
  trust_remote_code: true
  chat_template: "{% for message in messages %}..."

Testing

• Added unit tests for both file-based and inline chat templates
• All existing tests pass
• Code formatted with black via pre-commit

Fixes the need to modify source code to use custom chat templates during LoRA fine-tuning.