fix: correct possessive 'it's' to 'its' in model docstrings

[Ricardo-M-L]

Summary

• Fix it's → its (possessive form) in ~35 instances across ~32 model files
• The phrase "Check the superclass documentation for it's generic methods" should use "its" (possessive), not "it's" (contraction of "it is")
• Also fix "It's default parameters" and "It's parameters" in controlnet_xs.py

Test plan

• Docstring-only changes, no functional impact

🤖 Generated with Claude Code