[CodeGen] Fix gather fusion on vector distribute path

[pashu123]

Don't attach lowering config to the gather operation. Let it fuse with the consumer operation. This eliminates the creation of a temporary buffer. The change restricts element-wise fusion in the case of softmax with a gather-like operation. Fixes https://github.com/iree-org/iree/issues/21107

[hanhanW]

@pashu123 if this PR fixes the issue, you can add Fixes https://github.com/iree-org/iree/issues/21107 in the PR description. The issue will be closed when you land the PR.

[MaheshRavishankar]

@Groverkss can you review this please.