trunk/cd89b0d3bdd356f8a74cd80c8b2451f783e7ef37: [S658886] Revert D96769355 cublasLt workspace size change (#183923)

Summary:

D96769355 changed cublasLt workspace allocation from hardcoded 1MB to

unified workspace size (~8MB on A100, 32MB on H100/Blackwell) via

unified_cublas_and_lt_workspaces(). The FBCODE guard intended to make

this opt-in for Meta builds uses #if !defined(FBCODE), but FBCODE is

never defined (the correct macro is FBCODE_CAFFE2), so unified workspace

is always enabled.

This workspace size change causes correctness test failures (cosine

< 0.99) when combined with D101842780 (which re-enables bias fusion

by reverting D95083507). The larger workspace causes cuBLASLt to select

different algorithms, producing numerically different predictions.

Reverting the workspace sizing line to hardcoded 1024 KiB (1MB) while

leaving unified_cublas_and_lt_workspaces() and the rest of D96769355

untouched.

Test Plan:

Verified via cogwheel inplace correctness tests:

• parent(A) vs prod+graft(D)+revert(D96769355): PASS
  
  https://www.internalfb.com/servicelab/experiment/1153532713586286


• parent(A) vs candidateD+revert(D96769355 workspace line): PASS
  
  https://www.internalfb.com/servicelab/experiment/1517866206580079

SEV: S658886

Differential Revision: D105336951

Pull Request resolved: #183923

Approved by: https://github.com/eqy, https://github.com/atalman

Co-authored-by: Ruolin Zan [email protected]