Fix -Wnontrivial-memcall

[lydkim]

Summary:
The user-defined copy assignment operator made the type non-trivially copyable and just copied x with a self-assignment check, even though the copy constructor is defaulted. Since the struct only contains a single uint16_t, this diff defaults the copy assignment operator and fixes the -Wnontrivial-memcall warning introduced when building with llvm 21.
(fyi: This is code clean up - the flag -Wnontrivial-memcall is planned on being disabled anyways for nowT251075049)

Action failed: fbcode//gloo:gloo (cxx_compile allreduce_local.cc (pic))
Local command returned non-zero exit code 1
Reproduce locally: `env --chdir="$(buck2 root --kind project)" -- 'BUCK_SCRATCH_PATH=buck-out/v2/tmp/fbcode/d083cfbac224 ...<omitted>... g-info=buck-out/v2/gen/fbcode/gloo/__gloo__/output_artifacts/__objects__/allreduce_local.cc.pic.dwo' (run `buck2 log what-failed` to get the full command)`
stdout:
stderr:
fbcode/gloo/allreduce_local.cc:35:12: error: first argument in call to 'memcpy' is a pointer to non-trivially copyable type 'gloo::float16' [-Werror,-Wnontrivial-memcall]
   35 |     memcpy(ptrs_[i], ptrs_[0], bytes_);
      |            ^
fbcode/gloo/allreduce_local.cc:49:1: note: in instantiation of member function 'gloo::AllreduceLocal<gloo::float16>::run' requested here
   49 | INSTANTIATE_TEMPLATE(float16);
      | ^
fbcode/gloo/allreduce_local.cc:40:48: note: expanded from macro 'INSTANTIATE_TEMPLATE'
   40 | #define INSTANTIATE_TEMPLATE(T) template class AllreduceLocal<T>;
      |                                                ^
fbcode/gloo/allreduce_local.cc:35:12: note: explicitly cast the pointer to silence this warning
   35 |     memcpy(ptrs_[i], ptrs_[0], bytes_);
      |            ^
      |            (void*)
1 error generated.

Reviewed By: r-barnes

Differential Revision: D90615212

[meta-codesync]

@lydkim has exported this pull request. If you are a Meta employee, you can view the originating Diff in D90615212.