`gelu_tanh` should actually use `tanh`

[avik-pal]

Motivation and description

Currently gelu_tanh uses sigmoid which prevents us from pattern matching and fusing the gelu into gemm calls for dense layers. See EnzymeAD/Reactant.jl#1420 for details. cc @wsmoses

Possible Implementation

Rename the current gelu_tanh to gelu_sigmoid. Re-implement gelu_tanh to follow the original paper implementation