[CK_TILE] FMHA BWD Launcher Interface
## Motivation
Reduce memory usage; Be prepared to implement optimizations of reducing
nsplits in deterministic cases.
## Technical Details
This PR introduces a new launcher interface for the FMHA backward
operation, replacing direct function calls with a more structured
approach. The launcher encapsulates kernel dispatch logic and provides
access to computed metadata like the number of dQ acc splits.
**Changes:**
- Added `fmha_bwd_launcher` class that wraps kernel execution and
exposes `dq_acc_splits`
- Moved `fmha_bwd_traits` construction earlier in the execution flow to
support launcher initialization
- Refactored code generation to produce both legacy API and new launcher
constructor
## Test Plan
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## Test Result
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## Submission Checklist
- [x] Look over the contributing guidelines at
https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.
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