[hotfix] IsContiguous failed when tensor's size on first dimension is 1. (#2127)

* udp

* fix
Co-authored-by: Quan Gan <coin2028@hotmail.com>

src/runtime/ndarray.cc

@@ -137,12 +137,18 @@ bool NDArray::IsContiguous() const {
   CHECK(data_ != nullptr);
   if (data_->dl_tensor.strides == nullptr)
     return true;
-  for (int i = 0; i < data_->dl_tensor.ndim - 1; ++i) {
-    if (data_->dl_tensor.strides[i] !=
-        data_->dl_tensor.shape[i+1] * data_->dl_tensor.strides[i+1])
+
+  // See https://github.com/dmlc/dgl/issues/2118 and PyTorch's compute_contiguous() implementation
+  int64_t z = 1;
+  for (int64_t i = data_->dl_tensor.ndim - 1; i >= 0; --i) {
+    if (data_->dl_tensor.shape[i] != 1) {
+      if (data_->dl_tensor.strides[i] == z)
+        z *= data_->dl_tensor.shape[i];
+      else
         return false;
     }
-  return data_->dl_tensor.strides[data_->dl_tensor.ndim - 1] == 1;
+  }
+  return true;
 }
 
 NDArray NDArray::CreateView(std::vector<int64_t> shape,