[Relax] ONNX frontend using relax softplus operator (#18242)

Use relax softplus operator in onnx frontend

Author: MarcelDudek

python/tvm/relax/frontend/onnx/onnx_frontend.py

@@ -1698,7 +1698,8 @@ class Softplus(OnnxOpConverter):
     @classmethod
     def _impl_v1(cls, bb, inputs, attr, params):
         dtype = inputs[0].struct_info.dtype
-        return relax.op.log(relax.op.exp(inputs[0]) + relax.const(1, dtype=dtype))
+        threshold = 10.0 if dtype == "float16" else 20.0
+        return relax.op.nn.softplus(inputs[0], threshold=threshold)
 
 
 class Softsign(OnnxOpConverter):