[TOSA] Fix -Wdangling-gsl and -Wunused-variable in TosaToLinalg.cpp (NFC)

llvm-project/mlir/lib/Conversion/TosaToLinalg/TosaToLinalg.cpp:2376:9:
 error: object backing the pointer will be destroyed at the end of the full-expression [-Werror,-Wdangling-gsl]
        tensor::getMixedSizes(rewriter, loc, input_real);
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

llvm-project/mlir/lib/Conversion/TosaToLinalg/TosaToLinalg.cpp:2366:10:
 error: unused variable 'imag_el_ty' [-Werror,-Wunused-variable]
    auto imag_el_ty = cast<FloatType>(
         ^
2 errors generated.

Author: DamonFool

mlir/lib/Conversion/TosaToLinalg/TosaToLinalg.cpp

@@ -2363,7 +2363,7 @@ struct FFT2dConverter final : OpRewritePattern<FFT2dOp> {
 
     auto real_el_ty = cast<FloatType>(
         cast<ShapedType>(input_real.getType()).getElementType());
-    auto imag_el_ty = cast<FloatType>(
+    [[maybe_unused]] auto imag_el_ty = cast<FloatType>(
         cast<ShapedType>(input_imag.getType()).getElementType());
 
     assert(real_el_ty == imag_el_ty);
@@ -2372,8 +2372,7 @@ struct FFT2dConverter final : OpRewritePattern<FFT2dOp> {
     SmallVector<Value> dynamicSizes;
 
     // Get [N, H, W]
-    ArrayRef<OpFoldResult> dims =
-        tensor::getMixedSizes(rewriter, loc, input_real);
+    auto dims = tensor::getMixedSizes(rewriter, loc, input_real);
 
     SmallVector<int64_t, 3> staticSizes;
     dispatchIndexOpFoldResults(dims, dynamicSizes, staticSizes);