Require liger-kernel 0.8.2 and drop the SAPO warning filter

[albertvillanova]

This PR raises the liger-kernel floor to 0.8.2 in the liger and dev extras, replacing the !=0.8.1 exclusion, and removes the SAPO warning filter that the bump makes obsolete.

Fix #6767.
Fix #6435.

Warning

Draft: liger-kernel 0.8.2 is not released yet, so installation and CI will fail until it ships. Ready to merge as soon as it does.

Motivation

liger-kernel 0.8.1 crashes on pre-Ampere GPUs (CUDA compute capability < 8.0) in the fused linear cross-entropy backward pass, so it is currently excluded with !=0.8.1 (#6516, #6518). The upstream guard (linkedin/Liger-Kernel#1316) merged about 24 hours after 0.8.1 was tagged, so it ships in 0.8.2.

The SAPO IndexPutBackward0 warning filter is coupled to the same bump: its upstream fix (linkedin/Liger-Kernel#1274) is in 0.8.1, which we exclude, so 0.8.2 is the first non-excluded release that contains it.

Solution

Move the floor to >=0.8.2 rather than merely dropping the exclusion. A bare >=0.8.0 would permanently re-permit the broken release, and it would not guarantee the SAPO fix either.

Changes

• Require liger-kernel>=0.8.2 in the liger extra and in the dev extra
• Drop the now obsolete comments pointing at the upstream guard
• Remove the IndexPutBackward0 entry from the pytest filterwarnings list, along with its comment block