Make non-optional attribute parsers error on unset fields

[GleasonK]

What happened?

Some non-optional attributes have default values implemented in the parser (example), this makes for nonsense errors at verification time instead of parse time:

$ cat /tmp/t.mlir
func.func @gather(%arg0: tensor<3x4x2xf32>, %arg1: tensor<2x3x2xi64>) -> tensor<2x3x2x2xf32> {
  %0 = "stablehlo.gather"(%arg0, %arg1) {
    dimension_numbers = #stablehlo.gather<
      offset_dims = [2, 3],
      collapsed_slice_dims = [0],
      start_index_map = [1, 0],
      // index_vector_dim = 2
    >,
    slice_sizes = array<i64: 1, 2, 2>,
    indices_are_sorted = false
  } : (tensor<3x4x2xf32>, tensor<2x3x2xi64>) -> tensor<2x3x2x2xf32>
  return %0 : tensor<2x3x2x2xf32>
}

$ stablehlo-opt /tmp/t.mlir 
/tmp/t.mlir:2:8: error: 'stablehlo.gather' op inferred type(s) 'tensor<3x2x2x2xf32>' are incompatible with return type(s) of operation 'tensor<2x3x2x2xf32>'