Fix indefinite article and grammar typos in comments and docstrings (#194254)

No functional changes. Corrects "a"/"an" agreement in comments and docstrings across AOTAutograd, RPC, AOTI runtime, JIT, lazy tensors, the stable C shim, FSDP, DTensor context parallel, distributions, FX unification, ONNX export, and datapipes, along with a few misspellings (FunctionalizedRngRuntime, SimpleIREval) and subject-verb agreement in the unification utils note.

Test Plan: comment-only change; no tests run.

Authored with the assistance of an AI coding agent.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/194254
Approved by: https://github.com/aorenste