Small type typo in parameter_classes.h (v2.0.0)

[obandomarcos]

Hi!

First of all, I'd like to appreciate the neat work @matteo-ronchetti has been doing on Torch-Radon!

I wanted to report a small typo that arises a compilation error when building Torch-Radon v2.0.0. In parameter_classes.h, both get_block_dim and get_grid_size headers return a wrong type inside ExecCfg class. I believe they should return vec3 instead of dim3.

Regards.