[Unity][BugFix] Fix a bug in relax gelu_tanh computation

[rickzx]

The gelu_tanh computation (e.g. the gelu_new activation in huggingface API) is incorrect in the current relax code. This PR fixes the bug.

Correctness testing passes (which fails previously):

inp = torch.randn((2, 4), dtype=torch.float32)

def tanh_gelu(input):
    return 0.5 * input * (1.0 + np.tanh(np.sqrt(2.0 / np.pi) * (input + 0.044715 * torch.pow(input, 3.0))))

out1 = tanh_gelu(inp)

class TanhGelu(nn.Module):
    def __init__(self):
        pass

    def forward(self, x: nn.Tensor):
        return op.gelu(x, "tanh")

forward_spec = {"forward": {"x": spec.Tensor([2, 4], dtype="float32")}}
gelu = TanhGelu().jit(forward_spec)
out2 = gelu['forward'](inp)

assert torch.allclose(out1, out2)

Updated unit tests

[rickzx]

As @CharlieFRuan suggested, opened another PR in tvm/unity branch: apache/tvm#16188. Will close this one