Remove unused variables in gloo/test/reduce_scatter_test.cc

Summary:
LLVM-15 has a warning `-Wunused-but-set-variable` which we treat as an error because it's so often diagnostic of a code issue. Unused variables can compromise readability or, worse, performance.

This diff either (a) removes an unused variable and, possibly, it's associated code, or (b) qualifies the variable with `[[maybe_unused]]`, mostly in cases where the variable _is_ used, but, eg, in an `assert` statement that isn't present in production code.

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Reviewed By: bunnypak, dmm-fb

Differential Revision: D53011663

fbshipit-source-id: e7bdca967333e551ca2d38f4b60381eae8681f09

Author: r-barnes

gloo/test/reduce_scatter_test.cc

@@ -81,7 +81,6 @@ TEST_P(ReduceScatterTest, SinglePointer) {
   const auto contextSize = std::get<1>(GetParam());
   const auto dataSize = std::get<2>(GetParam());
   const auto fn = std::get<3>(GetParam());
-  const auto base = std::get<4>(GetParam());
 
   spawn(transport, contextSize, [&](std::shared_ptr<Context> context) {
     const auto contextRank = context->rank;