Implement inplace QKV fusion for the Wan-family transformers
All six `fuse_projections` overrides accepted `inplace` and ignored it, so
`fuse_qkv_projections(inplace=True)` on a Wan model quietly did a non-inplace
fusion and freed no memory. Two of them (SkyReelsV2, WanAnimate2) still had the
old signature and raised `TypeError` even on the plain no-argument call.
Each override now stores the split dims and drops the individual projections,
matching the base implementation. `WanAttnProcessor` and friends guarded the
image-context branch on `attn.add_k_proj is not None`, which breaks once that
module is deleted; they read `attn.added_kv_proj_dim` instead, which is set
from the config and always present.
The `unfuse_projections` overrides are gone. The base implementation does the
same removals and also reconstructs the split projections from the stored dims,
so inheriting it is what makes the round trip work.
`check_copies` compares only the body of a `# Copied from` block, not the `def`
line, so the stale signature in wan_animate_2 never showed up as a mismatch.
SkyReelsV2 also had no `AttentionTesterMixin` test class, so nothing exercised
its fusion path at all; add one.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
