corrections around data checker GPU and transfer optimisation for save_intermediate

[dkazanc]

pls ignore the name for the branch :)

1. It removes the insertion of data_checker (GPU) after save_intermediate_data method. Data is being checked before save_intermediate_data. So the order is more logical now:

    total_variation_PD (httomolibgpu)
    --- output check with data_checker --- (httomolibgpu)
    save_intermediate_data (httomo)
    calculate_stats (httomo)

compared to:

    total_variation_PD (httomolibgpu)
    save_intermediate_data (httomo)
    --- output check with data_checker --- (httomolibgpu)
    calculate_stats (httomo)

2. More major change around the transfer in SaveIntermediateFilesWrapper. Now the block can be transferred wholly to CPU, only if the method that follows is a CPU method. That saves of transferring from Device to Host twice. If the method after is a GPU method or there is no method then we only copy block to CPU in order to be saved, while the block itself remains on the GPU.

3. I also removed nan_to_num bit from calculate stats wrapper as all the data should be corrected with data checker beforehand.

One of the main reasons of doing this change is trying to understand the GPU transfers related numbers for the paper.
So to add more context, this was the result on the Blackwell node before the changes and this stuff around calculate_stats really confused me, like why is this the GPU kernel time, why it is so long as well.

And this after the changes I've made. calculate_stats is purely on the CPU as the block-data already converted beforehand in save_intermediate_data method. It is indeed faster (few seconds for this small data but when we scale up it will be more noticeable).

Checklist

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have made corresponding changes to the documentation
• I have added the user-release-note label in order to include this PR in the "Notable
  Changes for Users" section in release notes