message: Fix linux compilation error with Shape type conversion
  Summary:
  Linux couldn't do an automatic type conversion from F32 to F16.  This change
  uses an appropriate API for changing the type
  Test Plan: All existing tests passing on OS/X
  Reviewers: jamesn
diff: @@ -641,7 +641,7 @@ StatusOr<poplar::program::Program> CreateReductionNoConvert(
  const HloInstruction* reduction_in = reduction->operand(0)->operand(0);
  const HloInstruction* init = reduction->operand(1);
  - Shape shape_init_fp16 = ShapeUtil::MakeShape(F16, init->shape().dimensions());
  + Shape shape_init_fp16 = ShapeUtil::ChangeElementType(init->shape(), F16);
  // init has to be of type F16, there are 2 cases here
  poplar::Tensor init_val;
  