Refactor gelu_tanh to use tanh for compiler pattern matching

[Copilot]

The current gelu_tanh implementation uses sigmoid_fast, which prevents Reactant.jl from pattern matching and fusing GELU operations into GEMM calls (see EnzymeAD/Reactant.jl#1420).

Changes

• Reimplemented gelu_tanh to use the standard paper formula with tanh_fast:

  x/2 * (1 + tanh_fast(√(2/π) * (x + 0.044715x³)))

  This enables compiler pattern matching while maintaining mathematical correctness.

• Created gelu_sigmoid preserving the old sigmoid-based implementation for users who prefer it:

  x * sigmoid_fast(√(8/π) * x * (1 + 0.044715x²))

• Updated derivatives for both variants with correct chain rule application

• Maintained backward compatibility: gelu constant still points to gelu_tanh

Both implementations are mathematically equivalent (verified to machine precision) and produce identical outputs.

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Original prompt

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This section details on the original issue you should resolve

<issue_title>gelu_tanh should actually use tanh</issue_title>
<issue_description>### 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</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes gelu_tanh should actually use tanh #640

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[CarloLucibello]

@copilot explain this derivation