Implement GELU Activation Function (Approximation)

[MaximilianSchreff]

This PR introduces the Gaussian Error Linear Unit (GELU) activation function to SystemDS as a built-in operation. The implementation uses the widely adopted approximate formulation (https://arxiv.org/abs/1606.08415).

This PR is part of a series of PRs to support famous Transformer architectures in SystemDS. The GELU activation the most commonly used activation functions in models like BERT and GPT.

Includes

• Forward pass
• Backward pass

Testing:

Added two simple test cases comparing the forward pass and backward pass results against PyTorch's implementation for correctness.

• The tests validate:
  • Forward pass against PyTorch's torch.nn.functional.gelu.
  • Backward pass against PyTorch's torch.autograd.grad.

[phaniarnab]

Thanks @MaximilianSchreff. I approved the tests to run.

[phaniarnab]

Thanks for the patch @MaximilianSchreff. I will merge it in the next days.

[phaniarnab]

@MaximilianSchreff, this PR is merged now.