Try fix ck decoder compilation in fbcode (2/n)

[tenpercent]

What does this PR do?

Fixes compilation error in cpp extensions for decoder and splitk decoder.
A few issues were fixed

• fbcode has -Werror flag enabled, but OSS did not
• the kernel wrapper definitions hid the virtual functions defined at base invoker
• while hiding, the signature was also changed, so I had to add an extra casting step to make the compiler happy
• while testing the change I got spurious accuracy errors, which I got rid of by copying the args struct instead of using a reference, as it's allocated on stack somewhere and the existing way is not safe

Test plan

# pytest /xformers/tests/test_mem_eff_attention.py -k decoder (a few times)

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.
cc @jianyuh @qianfengz

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.72%. Comparing base (132da6a) to head (02d089c).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1012      +/-   ##
==========================================
- Coverage   60.73%   60.72%   -0.01%     
==========================================
  Files         113      113              
  Lines        9769     9777       +8     
==========================================
+ Hits         5933     5937       +4     
- Misses       3836     3840       +4     

Flag	Coverage Δ
Python	60.72% <ø> (-0.01%)	⬇️

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