viable/strict/1789752186: [MPS] Migrate `softplus` to Metal (#197000)

发生了什么

softplus and softplus_backward are structured TensorIterator ops whose only extra state is two scalars, so they fit the unary/binary-with-params Metal harness that elu already uses: two functors in ActivationKernel.metal plus stub registrations replace ~150 lines of MPSGraph. beta and threshold are carried as floats in a single non-templated SoftplusParams , since opmath is float for all three supported dtypes, matc…

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