[LLVMGPU] Add dense and paged attention runtime correctness tests (#24829)

## Summary

This PR adds a CUDA end-to-end runtime correctness test for paged-KV
attention as Stage 2 of
[#24766](https://github.com/iree-org/iree/issues/24766).

This change also fixes and enables the shared host-side attention
checker used by the existing dense-attention e2e tests. Before this
change, the reference softmax was numerically incorrect and the checker
generated expected results without comparing them against device output.

## Changes

- Adds CUDA paged-KV attention coverage with non-contiguous, distinct
key and value page tables, `iree_linalg_ext.gather`, and
`iree_linalg_ext.online_attention`.
- Makes the page-table dimension dynamic (`tensor<4x?xi64>`), matching
the runtime `NUM_PAGES` use case.
- Fixes the shared stable-softmax reference and makes it compare device
output with expected results.
- Refactors the paged checker to gather paged K/V into dense tensors and
reuse the shared attention reference and checker.
- Passes the f16-rounded `1 / sqrt(head_dim)` scale to both the
operation and host reference.

## Testing
Validated on NVIDIA RTX PRO 6000 Blackwell Server Edition

- All existing CPU dense-attention configurations.
- CUDA paged attention with 4 pages.
- The same compiled paged-attention VMFB with 2, 4, and 6 runtime pages.

Attention e2e tests use:

```text
--require_exact_results=false
--acceptable_fp_delta=0.01
```

---------

Signed-off-by: weimin023 <tnwilly@gmail.com>