[mlir][vector] Remove default value in populateVectorMultiReductionLoweringPatterns

[dcaballe]

Summary

Having a default value for the lowering strategy of the multi-reduction op has proven
to be unexpected by users. This patch is dropping the default value so that users have
to explicitly choose the lowering strategy to be applied.

mlir/include/mlir/Dialect/Vector/VectorRewritePatterns.h

 /// obtain a sequence of extract + vector.reduction + insert. This can further
 /// lower to horizontal reduction ops.
 /// - OneDimMultiReductionToTwoDim: for cases that reduce to 1-D vector<k>
 /// reduction (and are thus missing either a parallel or a reduction), we lift
 /// them back up to 2-D with a simple vector.shape_cast to vector<1xk> so that
 /// the other patterns can kick in, thus fully exiting out of the
 /// vector.multi_reduction abstraction.
 void populateVectorMultiReductionLoweringPatterns(
-    RewritePatternSet &patterns,
-    VectorMultiReductionLowering options =
-        VectorMultiReductionLowering::InnerParallel);
+    RewritePatternSet &patterns, VectorMultiReductionLowering options);
 
 /// Collects patterns to progressively lower vector contraction ops on high-D
 /// into low-D reduction and product ops.
 void populateVectorContractLoweringPatterns(
     RewritePatternSet &patterns,
     VectorTransformsOptions options = VectorTransformsOptions());
 
 /// Collect patterns to convert reduction op to vector.contract and fold