Move CrossReplicaSum from StableHLO to CHLO.

[subhankarshah]

Part 1 of Many

• Create a copy of stablehlo.cross-replica-sum in CHLO, including changes in .td and .cc files.

CrossReplicaSum is a special case of AllReduce where the reduction is a summation operation.
It is therefore decomposable to AllReduce. We intend to move the op to CHLO and lower it to AllReduce.

[burmako]

Thank you for your contribution! Since this PR proposes to modify the opset, it'll need to go through the RFC process before it can get merged. I'm finishing setting up the process and will follow up here later this week.

Also, I wanted to document our in-person discussion about what happens next if/when this pull request is approved. This is how I propose this should look like.

(M0) Current state:

• There's stablehlo.cross-replica-sum.
• There's mhlo.cross-replica.sum.
• stablehlo.cross-replica-sum => [stablehlo-legalize-to-hlo] => mhlo.cross-replica.sum.
• mhlo.cross-replica-sum => [hlo-legalize-to-stablehlo] => stablehlo.cross-replica.sum.

(M1) After this pull request (and a pending follow-on in MLIR-HLO):

• There's chlo.cross-replica-sum.
• chlo.cross-replica-sum => [chlo-legalize-to-hlo] => mhlo.all_reduce.
• There's stablehlo.cross-replica-sum.
• There's mhlo.cross-replica.sum.
• stablehlo.cross-replica-sum => [stablehlo-legalize-to-hlo] => mhlo.cross-replica.sum.
• mhlo.cross-replica-sum => [hlo-legalize-to-stablehlo] => stablehlo.cross-replica.sum.

(M2) Clean up MHLO (should be done shortly afterwards, because we don't provide stability guarantees for MHLO):

• There's chlo.cross-replica-sum.
• chlo.cross-replica-sum => [chlo-legalize-to-hlo] => mhlo.all_reduce.
• There's stablehlo.cross-replica-sum.
• stablehlo.cross-replica-sum => [stablehlo-legalize-to-hlo] => mhlo.all_reduce.

(M3) Clean up StableHLO (need to wait for the backward compatibility window)

• There's chlo.cross-replica-sum.
• chlo.cross-replica-sum => [chlo-legalize-to-hlo] => mhlo.all_reduce.

[GleasonK]

From a compatibility perspective, Migration Change. We will need to keep both Ops around, unless we figure out a way to use a generic Operation * for upgrade pass, before verifiers are run. I'll be investigating this in the near future.

(M0)

• Current state, no impact.

(M1)

• Verifier (pre-downgrade) blocks serialization of stablehlo.cross-replica-sum
• chlo.cross_replica_sum => [downgrade] => stablehlo.cross-replica-sum (remove after forward compatibility window)
• stablehlo.cross-replica-sum => [upgrade] => chlo.cross_replica_sum

(M2)

• No impact.

(M3)

• Remove upgrade pass, delete stablehlo.cross-replica-sum

[burmako]

Closing this pull request for now as discussed in #3 (comment). Will revisit once we pick up the associated ticket again.