message: Correct small EDSL op errors
  This combines:
  * Fix compiler error: comparison between signed and unsigned integer expressions
  * Correct name in sum op
diff: @@ -1334,7 +1334,7 @@ Value flip(const Value& value) {
  if (args[1].is_int()) {
  raw_axes.push_back(args[1].as_int());
  } else if (args[1].is_none()) {
  - for (int64_t i = 0; i < ndims; ++i) {
  + for (uint64_t i = 0; i < ndims; ++i) {
  raw_axes.push_back(i);
  }
  } else {
  @@ -2004,7 +2004,7 @@ Value sum(const Value& value) {
  IVLOG(1, "sum");
  auto args = value.as_tuple();
  if (args.size() != 3) {
  - throw std::runtime_error("mean expects 3 arguments");
  + throw std::runtime_error("sum expects 3 arguments");
  }
  auto I = args[0].as_tensor();
  