Sigmoid and softmax activation functions for `brulee_mlp()`

[christophscheuch]

Feature

Currently, brulee_mlp() supports "relu", "elu", "tanh", and "linear" activation functions. Would be great to also have "sigmoid" and "softmax" as they are also supported via torch::nn_sigmoid() and torch::nn_softmax(). I assume it just a few additional lines in the get_activation_fn(): https://github.com/tidymodels/brulee/blob/087129b0a71e63f16137934f89091b4db7fa4351/R/mlp-fit.R#L865C22-L865C22

[github-actions]

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