Add `tile_size` to arguments of function `build_centroids` in `uniform.py`

[msseibel]

I want to perform uniform sampling for my own dataset by first building centroids using build_centroids. The dimensions of my images are smaller than the hard_coded tile_size of 1024. Thus calc_tile_locations returns an empty list.

Fix:

1. Add argument tile_size to function build_centroids as it seems to be the entry point for building centroids
2. calc_tile_locations should not return empty lists. Perhaps raise an error. At least give some user feedback.

[msseibel]

After finding https://github.com/NVIDIA/semantic-segmentation/issues/106 I understand, that tile_size should be changed using the cfg.DATASET.CLASS_UNIFORM_TILE argument.

Still the bug described in the above mentioned issue must be fixed.