[quant] fill_ path for quantized tensors (pytorch#43303)

Summary: Pull Request resolved: pytorch#43303

Test Plan: Imported from OSS

Reviewed By: raghuramank100

Differential Revision: D23231947

Pulled By: z-a-f

fbshipit-source-id: fd5110ff15a073f326ef590436f8c6e5a2608324

Author: Zafar

aten/src/ATen/native/Fill.cpp

@@ -19,6 +19,13 @@ namespace {
 } // namspace
 
 Tensor& fill_out(Tensor& self, Scalar value) {
+  if (self.is_quantized()) {
+    at::Tensor out = at::ones(self.sizes()).to(kFloat) * value;
+    out = out.to(self.device());
+    // Trust the `copy_` to handle the quantization and the boundary chacks.
+    self.copy_(out);
+    return self;
+  }
   // When filling a number to 1-element CPU tensor, we want to skip
   // everything but manipulate data ptr directly.
   // Ideally this fast pass should be implemented in TensorIterator,

test/quantization/test_quantized_tensor.py

@@ -484,6 +484,35 @@ def test_qtensor_clone(self):
                 # Check to make sure the scale and zero_point has been copied.
                 self.assertEqual(q, q2)
 
+    def test_qtensor_fill(self):
+        numel = 10
+        scale = 0.5
+        zero_point = 10
+
+        ones = torch.ones(numel).to(torch.float)
+
+        types = [torch.qint8, torch.quint8, torch.qint32]
+        fills = [-1, 1, 2**32]  # positive, negative, overflow
+
+        # `fill_` uses `copy_(float)`, which doesn't support CUDA
+        device = 'cpu'
+        ones = ones.to(device)
+        for qtype, fill_with in itertools.product(types, fills):
+            q_filled = torch._empty_affine_quantized(
+                [numel], scale=scale, zero_point=zero_point, device=device,
+                dtype=qtype)
+            q_filled.fill_(fill_with)
+            int_repr = torch.quantize_per_tensor(ones * fill_with, scale,
+                                                 zero_point, qtype)
+            fill_with = int_repr.dequantize()
+            int_repr = int_repr.int_repr()
+
+            self.assertEqual(q_filled.int_repr(), int_repr)
+            self.assertEqual(q_filled.dequantize(), fill_with)
+            # Make sure the scale and zero_point don't change
+            self.assertEqual(q_filled.q_scale(), scale)
+            self.assertEqual(q_filled.q_zero_point(), zero_point)
+
     def test_qtensor_view(self):
         scale, zero_point, dtype = 1.0, 2, torch.uint8
         for device in get_supported_device_types():