dialects: extend the stencil lowerings

[PapyChacal]

This adds support for returning stencil.field from a func.func, and using stencil.fields as scf.for carried variables.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.03 🎉

Comparison is base (671d571) 86.89% compared to head (e363183) 86.92%.

❗ Current head e363183 differs from pull request most recent head 8b4c03a. Consider uploading reports for the commit 8b4c03a to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1028      +/-   ##
==========================================
+ Coverage   86.89%   86.92%   +0.03%     
==========================================
  Files         125      125              
  Lines       19339    19355      +16     
  Branches     2933     2940       +7     
==========================================
+ Hits        16804    16825      +21     
+ Misses       2033     2030       -3     
+ Partials      502      500       -2     

Impacted Files	Coverage Δ
xdsl/dialects/experimental/stencil.py	95.45% <100.00%> (+0.01%)	⬆️
.../transforms/experimental/ConvertStencilToLLMLIR.py	90.55% <100.00%> (+0.64%)	⬆️

... and 3 files with indirect coverage changes

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[meshtag]

I had a first look at the implementation and this looks very good!
I wish to have a deeper look as soon as I get more time.

Do you think it makes sense to split this PR in two PRs, one for implementing changes in return_target_analysis (due to recent modification in stencil types) and another small one for adding support related to scf.for variable propagation?

[AntonLydike]

🚀 I think it's pretty good already, some minor nitpicks and docstrings would be nice tho!

[PapyChacal]

I had a first look at the implementation and this looks very good!
I wish to have a deeper look as soon as I get more time.

Do you think it makes sense to split this PR in two PRs, one for implementing changes in return_target_analysis (due to recent modification in stencil types) and another small one for adding support related to scf.for variable propagation?

I think you had a deep enough look to raise this good point 😉
To answer, as a best practice, it would be nice, but we're back in iteration mode, so I wouldn't bother unless you really want it, as a fellow stencil owner 🙂

[meshtag]

To answer, as a best practice, it would be nice, but we're back in iteration mode, so I wouldn't bother unless you really want it, as a fellow stencil owner slightly_smiling_face

No strict requirement from my side

LGTM aside from some minor comments and a possibly trivial question ;)