[Codegen] Move codegen pipelines to work on online_attention (#24110)

This patch moves all codegen pipelines to move to working on
iree_linalg_ext.online_attention instead of iree_linalg_ext.attention.

iree_linalg_ext.attention is really a legacy operation for codegen and
not really the best thing to form dispatches. The main reason is that
online_attention returns the "attention state" instead of the attention
result. The "attention state" can be used to do split-k and is also
useful for some frontends (as logsumexp).

The main changes are:

- Add/Move convertAttentionToOnlineAttention in the configuration part
of the pipieline.
- Move KernelConfig.cpp to work on online_attention instead of
attention.

Pipelines already handle online_attention properly, so there should be
no major changes there.

Assisted-By: Codex