make the empty parens after level constructors optional (NVIDIA#2750)

For example, what before needed to be written as:

```cpp
cudax::make_hierarchy(cudax::block_dims<block_size>(), cudax::grid_dims<grid_size>())
```

can now be written as:

```cpp
cudax::make_hierarchy(cudax::block_dims<block_size>, cudax::grid_dims<grid_size>)
```

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Co-authored-by: pciolkosz <pciolkosz@nvidia.com>