[SYCL][COMPAT] Add default constructor for dim3

[aacostadiaz]

This PR add a default constructor for the Dim3 class.

[Alcpz]

Could you change the default constructor to set dimensions to 0? I think its better to force the user to always specify their workgroup size.

[Alcpz]

You're right. I've removed your comment requesting the merge. Removing const from the attributes would require modifying all of our constexpr constructors and would reduce a bit of performance. We haven't encountered a case where we needed to modify the original dim3 object, so if there is no alternative, feel free to include the changes to the header that would make this work. This would include modifying the documentation and maybe creating a test

[joeatodd]

Closing for now. Please re-open as/when required @aacostadiaz