Support All keras activations functions

[EmilHvitfeldt]

Follows the style as seen in tidymodels/brulee@88d6002#diff-c28a6671db86faf0c62ceedbd8e305dff10d29d4ebe346e02877b429ee4eacbc

To close #1127

library(tidymodels)

model <- 
  mlp(activation = "selu") %>%
  set_engine("keras") %>%
  set_mode("classification")


fit(model, class ~ ., sim_classification(1000))
#> parsnip model object
#> 
#> Model: "sequential"
#> ________________________________________________________________________________
#>  Layer (type)                       Output Shape                    Param #     
#> ================================================================================
#>  dense (Dense)                      (None, 5)                       80          
#>  dense_1 (Dense)                    (None, 2)                       12          
#> ================================================================================
#> Total params: 92 (368.00 Byte)
#> Trainable params: 92 (368.00 Byte)
#> Non-trainable params: 0 (0.00 Byte)
#> ________________________________________________________________________________

[EmilHvitfeldt]

only failure is Failure ('test-fit_interfaces.R:168:3'): overhead of parsnip interface is minimal (#1071) ──

[topepo]

Thanks! Looks good. I added some tests.

Can you go over to dials and update the valid activation list to include these and the ones from brulee?

[github-actions]

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