Add as_quantized_tensor (#20740)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20740

Provide a way to assemble quantized Tensor from int8 Tensor, scale and zero point.

Differential Revision: D15232416

fbshipit-source-id: c3a3d9d7214b1dc569214c019440c2779fbd063b

aten/src/ATen/native/native_functions.yaml

@@ -2573,6 +2573,10 @@
   dispatch:
     QuantizedCPU: int_repr_quant
 
+- func: _per_tensor_affine_qtensor(Tensor self, float scale, int zero_point) -> Tensor
+  dispatch:
+    CPU: per_tensor_affine_qtensor_cpu
+
 # to(Device) must not exist because all constructors of Device also works for
 # TensorOptions. Otherwise, an ambiguity error is thrown.
 # See NOTE [ TensorOptions Constructors ].

aten/src/ATen/native/quantized/QTensor.cpp

@@ -60,5 +60,17 @@ Tensor int_repr_quant(const Tensor& self) {
   return dst;
 }
 
+Tensor per_tensor_affine_qtensor_cpu(const Tensor& self, double scale, int64_t zero_point) {
+  Tensor dst = at::_empty_affine_quantized(self.sizes(), self.options().dtype(toQIntType(self.scalar_type())), scale, zero_point);
+  AT_DISPATCH_QINT_TYPES(dst.scalar_type(), "per_tensor_affine_qtensor", [&]() {
+    underlying_t* self_data = self.data<underlying_t>();
+    underlying_t* dst_data = reinterpret_cast<underlying_t *>(dst.data<scalar_t>());
+    if (self.numel() > 0) {
+      memcpy(dst_data, self_data, self.numel());
+    }
+  });
+  return dst;
+}
+
 } // namespace native
 } // namespace at

test/test_torch.py

@@ -2798,8 +2798,16 @@ class _TestTorchMixin(object):
         zero_point = 10
         val = 100
         numel = 10
-        q = torch._empty_affine_quantized(numel, dtype=torch.quint8, scale=scale, zero_point=zero_point)
-        # TODO: check dequantized values?
+        q = torch._empty_affine_quantized([numel], scale=scale, zero_point=zero_point, dtype=torch.quint8)
+        self.assertEqual(scale, q.q_scale())
+        self.assertEqual(zero_point, q.q_zero_point())
+
+        # create Tensor from uint8_t Tensor, scale and zero_point
+        int_tensor = torch.randint(0, 100, size=(10,), dtype=torch.uint8)
+        q = torch._per_tensor_affine_qtensor(int_tensor, scale, zero_point)
+        self.assertEqual(int_tensor, q.int_repr())
+        self.assertEqual(scale, q.q_scale())
+        self.assertEqual(zero_point, q.q_zero_point())
 
     def test_qtensor_dtypes(self):
         r = np.random.rand(3, 2) * 2 - 4