Repository: Project-MONAI/MONAI

Issue:
ToTensor Device
**Is your feature request related to a problem? Please describe.**
Currently when converting from other formats that can reside on gpu like cupy to torch tensor, using `ToTensor`, it will create the tensor on the cpu, and then it should be sent to gpu using `ToDevice`. This will create unnecessary between cpu and gpu and reduce the performance. A solution to that is to have `ToTensor` accept device parameter and directly create the tensor on that machine, and in this case on gpu.



Discussion:

Resolving patch:
diff --git a/monai/transforms/utility/array.py b/monai/transforms/utility/array.py
--- a/monai/transforms/utility/array.py
+++ b/monai/transforms/utility/array.py
@@ -354,19 +354,17 @@ class EnsureType(Transform):
 
     Args:
         data_type: target data type to convert, should be "tensor" or "numpy".
+        device: for Tensor data type, specify the target device.
 
     """
 
     backend = [TransformBackends.TORCH, TransformBackends.NUMPY]
 
-    def __init__(self, data_type: str = "tensor") -> None:
-        data_type = data_type.lower()
-        if data_type not in ("tensor", "numpy"):
-            raise ValueError("`data type` must be 'tensor' or 'numpy'.")
-
-        self.data_type = data_type
+    def __init__(self, data_type: str = "tensor", device: Optional[torch.device] = None) -> None:
+        self.data_type = look_up_option(data_type.lower(), {"tensor", "numpy"})
+        self.device = device
 
-    def __call__(self, data: NdarrayOrTensor) -> NdarrayOrTensor:
+    def __call__(self, data: NdarrayOrTensor):
         """
         Args:
             data: input data can be PyTorch Tensor, numpy array, list, dictionary, int, float, bool, str, etc.
@@ -375,7 +373,7 @@ def __call__(self, data: NdarrayOrTensor) -> NdarrayOrTensor:
                 if applicable.
 
         """
-        return convert_to_tensor(data) if self.data_type == "tensor" else convert_to_numpy(data)  # type: ignore
+        return convert_to_tensor(data, device=self.device) if self.data_type == "tensor" else convert_to_numpy(data)
 
 
 class ToNumpy(Transform):
diff --git a/monai/transforms/utility/dictionary.py b/monai/transforms/utility/dictionary.py
--- a/monai/transforms/utility/dictionary.py
+++ b/monai/transforms/utility/dictionary.py
@@ -486,16 +486,23 @@ class EnsureTyped(MapTransform, InvertibleTransform):
 
     backend = EnsureType.backend
 
-    def __init__(self, keys: KeysCollection, data_type: str = "tensor", allow_missing_keys: bool = False) -> None:
+    def __init__(
+        self,
+        keys: KeysCollection,
+        data_type: str = "tensor",
+        device: Optional[torch.device] = None,
+        allow_missing_keys: bool = False,
+    ) -> None:
         """
         Args:
             keys: keys of the corresponding items to be transformed.
                 See also: :py:class:`monai.transforms.compose.MapTransform`
             data_type: target data type to convert, should be "tensor" or "numpy".
+            device: for Tensor data type, specify the target device.
             allow_missing_keys: don't raise exception if key is missing.
         """
         super().__init__(keys, allow_missing_keys)
-        self.converter = EnsureType(data_type=data_type)
+        self.converter = EnsureType(data_type=data_type, device=device)
 
     def __call__(self, data: Mapping[Hashable, NdarrayOrTensor]) -> Dict[Hashable, NdarrayOrTensor]:
         d = dict(data)


Test patch:
diff --git a/tests/test_ensure_type.py b/tests/test_ensure_type.py
--- a/tests/test_ensure_type.py
+++ b/tests/test_ensure_type.py
@@ -36,7 +36,7 @@ def test_single_input(self):
             test_datas.append(test_datas[-1].cuda())
         for test_data in test_datas:
             for dtype in ("tensor", "numpy"):
-                result = EnsureType(data_type=dtype)(test_data)
+                result = EnsureType(data_type=dtype, device="cpu")(test_data)
                 self.assertTrue(isinstance(result, torch.Tensor if dtype == "tensor" else np.ndarray))
                 if isinstance(test_data, bool):
                     self.assertFalse(result)
diff --git a/tests/test_ensure_typed.py b/tests/test_ensure_typed.py
--- a/tests/test_ensure_typed.py
+++ b/tests/test_ensure_typed.py
@@ -75,7 +75,7 @@ def test_dict(self):
             "extra": None,
         }
         for dtype in ("tensor", "numpy"):
-            result = EnsureTyped(keys="data", data_type=dtype)({"data": test_data})["data"]
+            result = EnsureTyped(keys="data", data_type=dtype, device="cpu")({"data": test_data})["data"]
             self.assertTrue(isinstance(result, dict))
             self.assertTrue(isinstance(result["img"], torch.Tensor if dtype == "tensor" else np.ndarray))
             torch.testing.assert_allclose(result["img"], torch.as_tensor([1.0, 2.0]))