Templatise `Fill_cpu()` and `Fill_gpu()` and increase block size

[IvanaGyro]

This PR is a task listed in #500.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 18.00%. Comparing base (72453ca) to head (eed01c2). Report is 50 commits behind head on dev-master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev-master #507 +/- ## ============================================== + Coverage 16.60% 18.00% +1.39% ============================================== Files 221 221 Lines 48490 52808 +4318 Branches 20272 19698 -574 ============================================== + Hits 8053 9508 +1455 - Misses 36143 38778 +2635 - Partials 4294 4522 +228 ```

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