[mlir][GPU] treat the absence of workgroup attributes correctly

[ftynse]

Summary

The helper function in GPUFuncOp incorrectly assumed the workgroup
attribution attribute is always present. Instead, treat its absence as
if its value was zero, i.e., no workgroup attributions are specified.

Closes #58045.

mlir/include/mlir/Dialect/GPU/IR/GPUOps.td

let extraClassDeclaration = [{
     /// it is intended to be launched from host.
     bool isKernel() {
       return (*this)->getAttrOfType<UnitAttr>(
           GPUDialect::getKernelFuncAttrName()) != nullptr;
     }
 
     /// Returns the number of buffers located in the workgroup memory.
     unsigned getNumWorkgroupAttributions() {
-      return (*this)->getAttrOfType<IntegerAttr>(
-          getNumWorkgroupAttributionsAttrName()).getInt();
+      auto attr = (*this)->getAttrOfType<IntegerAttr>(
+          getNumWorkgroupAttributionsAttrName());
+      return attr ? attr.getInt() : 0;
     }
 
     /// Returns a list of block arguments that correspond to buffers located in
     /// the workgroup memory
     ArrayRef<BlockArgument> getWorkgroupAttributions() {
       auto begin =
           std::next(getBody().args_begin(), getFunctionType().getNumInputs());
       auto end = std::next(begin, getNumWorkgroupAttributions());

mlir/test/Dialect/GPU/ops.mlir

module attributes {gpu.container_module} {
 
   // CHECK-LABEL: func @set_default_device
   func.func @set_default_device(%arg0: i32) {
     // CHECK: gpu.set_default_device
     gpu.set_default_device %arg0
     return
   }
 }
+
+// Just check that this doesn't crash.
+gpu.module @module {
+  "gpu.func"() ({
+    gpu.return
+  }) {function_type = () -> (), sym_name = "func"} : () -> ()
+}