Replace incorrect possessive "its" with the contraction "it's" (it is)
in comments and docstrings across multiple files where the intended
meaning is "it is" or "it has" rather than the possessive form.

Authored with Claude (typo_terminator2).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/185720
Approved by: https://github.com/zou3519, https://github.com/cyyever