fix(seedvr): restore distributed ops exports

[GACLove]

Summary

• restore the four SeedVR distributed-op exports removed by support sp for seedvr2 #1345
• route existing SeedVR inference imports to the sequence-parallel implementations
• preserve single-GPU no-op behavior

Validation

• SeedVR source imports pass in the CUDA runtime
• single-GPU semantics verified for all four exports
• full SeedVR2 3B BF16 inference passed (305-frame input, 1920x1080 output)
• full SeedVR2 3B FP8 inference passed (305-frame input, 1920x1080 output)

Without this change, both BF16 and FP8 fail before loading weights with ImportError for gather_heads_scatter_seq.