[Frontend][ONNX] add onnx Mish operator (#15415)

* added mish operator to onnx frontend

* linter reformat

* fixed lint issues as linter failed on the CI

* added test for mish operator

* added test for mish operator

* pytest skip since ONNX Runtime in CI does not support domain version 18

* linter format

python/tvm/relay/frontend/onnx.py

@@ -1161,6 +1161,24 @@ def _impl_v1(cls, inputs, attr, params):
         return Gelu._impl_v1([inp], attr, params)
 
 
+class Mish(OnnxOpConverter):
+    """Operator converter for Mish from Microsoft onnxruntime contrib opset.
+
+    mish(x) = x * tanh(softplus(x)) = x * tanh(ln(1 + e^{x}))
+    """
+
+    @classmethod
+    def _impl_v18(cls, inputs, attr, params):
+        x = inputs[0]
+        # Declare const
+        const_dtype = infer_type(x).checked_type.dtype
+        one = _expr.const(1.0, dtype=const_dtype)
+
+        # Compute Mish
+        term1 = _op.log(one + _op.exp(x))
+        return _op.multiply(x, _op.tanh(term1))
+
+
 class LayerNormalization(OnnxOpConverter):
     """Operator converter for LayerNormalization from Microsoft onnxruntime contrib opset."""
 
@@ -6536,6 +6554,7 @@ def _get_convert_map(opset):
         "Gelu": Gelu.get_converter(opset),
         "FastGelu": FastGelu.get_converter(opset),
         "BiasGelu": BiasGelu.get_converter(opset),
+        "Mish": Mish.get_converter(opset),
         "LayerNormalization": LayerNormalization.get_converter(opset),
         # TODO: We need a better way to handle different domains, in case
         # of name collisions. EmbedLayerNormalization, SkipLayerNormalization, and Attention

tests/python/frontend/onnx/test_forward.py

@@ -2489,6 +2489,15 @@ def selu_x(x, alpha, gamma):
     )
 
 
+@pytest.mark.skip("Currently ONNX Runtime in CI does not support domain version of 18")
+@tvm.testing.parametrize_targets
+def test_mish(target, dev):
+    def mish_x(x):
+        return x * np.tanh(np.log1p(np.exp(x)))
+
+    _test_onnx_op_elementwise(target, dev, (2, 4, 5, 6), mish_x, {}, "float64", "Mish", {})
+
+
 @tvm.testing.parametrize_targets
 def test_prelu(target, dev):
     """test_prelu"""