ciflow/trunk/186519: c10/util/intrusive_ptr.h: add [[nodiscard]] to query methods

Summary: Add [[nodiscard]] to the pure-query methods on both intrusive_ptr and weak_intrusive_ptr. Silently discarding the result of get(), defined(), use_count(), weak_use_count(), unique(), is_uniquely_owned(), or expired() is always a logic error. The get() annotation is particularly important — calling ptr.get() and ignoring the return is a common mistake that this makes a compile error.

Test Plan: buck2 test fbcode//caffe2/c10/test:core_tests — 86 tests pass, 0 failures.

Differential Revision: D107127349