Introduces cutlass::epilogue::thread::Snake, a two-operand activation
functor implementing Snake_a(x) = x + (1/a) * sin^2(a*x) from
Ziyin et al. 2020 (arXiv:2006.08195). The per-channel learnable
frequency `a` flows through an EVT child (e.g. Sm90RowBroadcast),
composing into Sm90EVT<Sm90Compute<Snake, ...>, x_node, alpha_node>
for fused GEMM+Snake epilogues used in neural vocoders.
Adds unit tests in test/unit/epilogue/thread/activation.cu covering
f32 and bf16 paths, validated against float64 reference goldens.
Closes#3141
2e56847d72