feat: native GroupNorm backward affine path (scoped)

[cafeTechne]

Summary:

• add native GroupNorm forward/backward operator hooks and SymInt signatures
• implement affine gradients for GroupNorm backward in native path
• add tracked regression coverage test_group_norm.py and GroupNorm test entry in test_op.py
• remove unrelated BCE/CMake changes from branch scope

Why this PR:

• This is intentionally scoped to GroupNorm behavior only; build/toolchain fixes were split into a separate PR for clean review.

Dependency:

• fix: unblock Windows/OpenCL build environment for feature validation #4 (build-env/toolchain baseline).

[cafeTechne]

Revalidated with build-env baseline + OpenCL pointwise/codegen fixes applied: ests/test_group_norm.py passes on OCL (forward + dx + affine grads within thresholds).