Remove Halide op's dependence on loop iters having same name + documenting limitations

[skeqiqevian]

Make Halide compute_at work without depending on variable names being equal. Also added some more documentation about current limitations.

[codecov[bot]]

Codecov Report

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

Project coverage is 86.99%. Comparing base (cc41f2c) to head (a85b03e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #649 +/- ## ========================================== - Coverage 86.99% 86.99% -0.01% ========================================== Files 94 94 Lines 22015 22022 +7 ========================================== + Hits 19153 19159 +6 - Misses 2862 2863 +1 ```

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