[CrashFix] Make the dst tensor contiguous when copying from metal

Summary: Somehow the destination tensor becomes incontiguous when copying from Metal. We need to call `.contiguous()` explicitly. See the crash log - https://www.internalfb.com/intern/logview/details/facebook_ios_crashes/1d865405fbc1a45f9517470906c9ec08/

Test Plan:
- verify the crash
- Sandcastle CIs

Reviewed By: dreiss

Differential Revision: D25502884

fbshipit-source-id: 46ee720bf6b6658e51cb56a4e4c16ce121eeabc7

aten/src/ATen/native/metal/mpscnn/MPSCNNOps.mm

@@ -608,7 +608,9 @@ Tensor copy_to_host(const Tensor& input) {
   MPSImage* X = imageFromTensor(input);
   MetalCommandBuffer* commandBuffer = commandBufferFromInputTensor(input);
   auto&& sizes = [X sizes];
-  MetalTensor mt{sizes};
+  auto dummy = at::zeros(input.sizes()).contiguous();
+  auto strides = dummy.strides();
+  MetalTensor mt{sizes, strides.vec()};
   mt.texture()->setCommandBuffer(commandBuffer);
   mt.texture()->allocateTextureStorage(sizes);
   MPSImage* Y = imageFromMetalTensor(mt);