[CK] Add flash_attn tests
## Motivation
Add CI support for running
[flash-attention](https://github.com/ROCm/flash-attention) tests against
CK, similar to existing AITER and PyTorch downstream test pipelines.
## Technical Details
### New: `Dockerfile.fa`
A new Dockerfile that builds a flash-attention test image on top of a
ROCm PyTorch base image. It:
- Sparse-checkouts CK from `rocm-libraries` (or clones directly from
`ROCm/composable_kernel`)
- Clones and builds `flash-attention` with CK as the backend
- Supports configurable `FA_BRANCH`, `CK_FA_BRANCH`, and `GPU_ARCHS`
build args
### Updated: `Jenkinsfile`
**buildDocker refactor:**
- Extracted `buildAndPushDockerImage()` helper that handles both "check
if exists, skip" and "force build, push" logic, eliminating the
duplicated try/catch blocks
- Split monolithic `buildDocker()` into `buildDockerBase()`,
`buildDockerPytorch()`, `buildDockerAiter()`, and new `buildDockerFa()`
- Each downstream docker build now runs unconditionally within its
respective guard (`RUN_PYTORCH_TESTS`, `RUN_AITER_TESTS`,
`RUN_FA_TESTS`)
- Image digests are stored in env vars (`CK_BASE_IMAGE`,
`CK_PYTORCH_IMAGE`, `CK_AITER_IMAGE`, `CK_FA_IMAGE`) for use in
downstream stages
**run_downstream_tests refactor:**
- Merged `run_aiter_tests()` and `run_pytorch_tests()` into a single
generic `run_downstream_tests(conf)` that accepts `image`,
`timeoutHours`, and `execute_cmds`
- Test commands for each downstream target are declared as top-level
lists (`RUN_PYTORCH_TESTS_CMDS`, `RUN_AITER_TESTS_CMDS`,
`RUN_FA_TESTS_CMDS`)
**Pipeline stages:**
- Merged "Run Pytorch Tests" and "Run AITER Tests" into a single "Run
Downstream Tests" parallel stage
- Added two new FA test stages: "Run FA Tests on gfx942" and "Run FA
Tests on gfx950"
- Added new pipeline parameters: `RUN_FA_TESTS`, `fa_base_docker`,
`fa_branch`, `ck_fa_branch`
- `ck_pytorch_branch` and `ck_aiter_branch` now default to the current
branch instead of hardcoded `develop`
- CRON schedule at 13:00 now also triggers `RUN_FA_TESTS=true`
## Test Plan
- [x] Trigger pipeline manually with `RUN_FA_TESTS=true` on gfx942 and
gfx950 nodes
- [x] Verify existing AITER and PyTorch test stages are unaffected
- [x] Verify `buildAndPushDockerImage` correctly skips rebuild when
image already exists (with `BUILD_DOCKER=false`)
## Test Result
<!-- Briefly summarize test outcomes. -->
## Submission Checklist
- [x] Look over the contributing guidelines at
https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.
