StableHLO CAPI requires building upstream dialects

[GleasonK]

Request description

Currently CAPI builds linalg and arith, etc -- but most users of CAPI (frameworks) only want StableHLO+CHLO+Serialization. Would be great to offer a stablehlo_dialect_capi which only has core APIs.

One possible separation direction:

• stablehlo_dialect_capi: Just StableHLO+CHLO+Serialization APIs, these are the things useful for PJRT frameworks.
• stablehlo_unified_capi: A separate target which builds the dialect CAPI along with the transformation passes / reference interpreter / other useful APIs, likely will be the target used for StableHLO python bindings

This will probably require splitting the serialization APIs into a separate file StablehloDialectApi.h and StablehloUnifiedApi.h. The following functions should go into the dialect API: https://github.com/openxla/stablehlo/blob/main/stablehlo/integrations/c/StablehloApi.h#L23-L117

The reference API and pass registration should go into the unified API target:
https://source.corp.google.com/piper///depot/google3/third_party/stablehlo/stablehlo/integrations/c/StablehloPasses.h

Looking at the CAPI deps today, I'm surprised that this requires upstream dialects, so definitely something to look into.

(Noted by @steeve at recent OpenXLA community meeting)

[cerisier]

Hey @GleasonK,

I'm having a go at this and am nearly ready to submit a draft PR.
Before submitting anything, I would love your thoughts on a few points:

1. API Export vs Re-export

Basically, either:

1. stablehlo_unified_capi target exports StablehloDialectApi.h as well as StablehloUnifiedApi.h (in the hdrs attribute) (but then downstreams users like python bindings have to import the 2 of them)
2. StablehloUnifiedApi.h re-exports StablehloDialectApi.h which becomes the only header to import.

2. Common deps between new StablehloUnifiedApi and StablehloDialectApi

Currently, stablehlo_capi compiles StablehloAttributes, StablehloDialect and StablehloTypes and exports their respective headers which will likely needed by users of the new stablehlo_dialect_capi.

I have several options:

1. Split those further into multiple targets and depend on them on both stablehlo_unified_capi and stablehlo_dialect_capi targets. (But then both targets need to either re-rexports the headers or exports them for manual inclusion downstream).
2. Have stablehlo_dialect_capi depend on them and stablehlo_unified_capi depend on stablehlo_dialect_capi.
3. Have both stablehlo_unified_capi and stablehlo_dialect_capi compile those files and exports their headers.

3. Backward compat for target names

Should I expose aliases so that stablehlo_capi points to stablehlo_unified_capi to preserve backward compat ?

If confusing, I can go ahead and submit what I have and we start from there ?

Thank you in advance 🙏