[SYCL] Renaming fixed_size_group to chunk

[AndreiZibrov]

First part of changes in order of https://github.com/intel/llvm/pull/14604: fixed_size_group -> chunk

[steffenlarsen]

I am concerned about doing the changes in https://github.com/intel/llvm/pull/14604 gradually. I do not mind having multiple patches for them, but they should go in around the same time. We also need to make sure we coordinate these changes with changes to https://github.com/KhronosGroup/SYCL-CTS/tree/SYCL-2020/tests/extension/oneapi_non_uniform_groups.