fix: disable_mmap_safetensors not defined in SDXL TI training

[kohya-ss]

closes #2214

[Copilot]

Pull Request Overview

This PR fixes a bug where the --disable_mmap_load_safetensors argument was not available in SDXL textual inversion training, addressing issue #2214.

• Enables proper SDXL training argument parsing by calling add_sdxl_training_arguments
• Standardizes parameter naming from camelCase to snake_case for consistency
• Updates function calls to use the corrected parameter names

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
sdxl_train_textual_inversion.py	Uncomments and fixes the call to add SDXL training arguments, updates parameter naming
library/sdxl_train_util.py	Adds conditional parameter support and standardizes naming convention

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