NNAPI: quant logistic fix (#70847)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70847

NNAPI needs a fixed zero point and scale for sigmoid (logistic)
ghstack-source-id: 146555935

Test Plan: LIBNEURALNETWORKS_PATH="/path/to/libneuralnetworks.so" pytest test/test_nnapi.py

Reviewed By: dreiss

Differential Revision: D33237918

fbshipit-source-id: 05ef3a81bf1589ad44b599a19bce4066531c432b

test/test_nnapi.py

@@ -286,6 +286,10 @@ class TestNNAPI(TestCase):
                             return torch.sigmoid(arg)
                         raise Exception("Bad op")
                 self.check(UnaryModule(), torch.tensor([-1.0, 1.0]))
+                self.check(
+                    UnaryModule(),
+                    qpt(torch.tensor([-1.0, 1.0]), 1. / 256, 0),
+                )
 
     def test_pointwise_binary(self):
         for op in ["add", "sub", "mul", "div"]:

torch/backends/_nnapi/serializer.py

@@ -1267,7 +1267,16 @@ class _NnapiSerializer(object):
         assert node.outputsSize() == 1
 
         in_id, in_oper = self.get_tensor_operand_by_jitval(node.inputsAt(0))
-        out_id = self.add_tensor_operand(node.outputsAt(0), in_oper)
+
+        out_oper = in_oper
+        if opcode == NNAPI_OperationCode.LOGISTIC:
+            # NNAPI docs: For ANEURALNETWORKS_TENSOR_QUANT8_ASYMM, the scale
+            # must be 1.f / 256 and the zeroPoint must be 0.
+            # https://fburl.com/h52stoog
+            if in_oper.op_type == NNAPI_OperandCode.TENSOR_QUANT8_ASYMM:
+                out_oper = in_oper._replace(zero_point=0, scale=1.0 / 256)
+
+        out_id = self.add_tensor_operand(node.outputsAt(0), out_oper)
 
         for idx, dim in enumerate(in_oper.shape):
             if dim == 0: