[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.