Fix lost compatibility with custom datatypes implementing .to (#2335)

* generalize data transfer

* added test

* update docs

* fix spelling error

* changelog

* update docs

CHANGELOG.md

@@ -28,6 +28,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 - Fixed `average_precision` metric ([#2319](https://github.com/PyTorchLightning/pytorch-lightning/pull/2319))
 
+- Fixed lost compatibility with custom datatypes implementing `.to` ([#2335](https://github.com/PyTorchLightning/pytorch-lightning/pull/2335))
+
 ## [0.8.1] - 2020-06-19
 
 ### Fixed

pytorch_lightning/core/hooks.py

@@ -204,7 +204,7 @@ def transfer_batch_to_device(self, batch: Any, device: torch.device) -> Any:
 
         The data types listed below (and any arbitrary nesting of them) are supported out of the box:
 
-        - :class:`torch.Tensor`
+        - :class:`torch.Tensor` or anything that implements `.to(...)`
         - :class:`list`
         - :class:`dict`
         - :class:`tuple`

pytorch_lightning/utilities/apply_func.py

@@ -1,3 +1,4 @@
+from abc import ABC
 from collections import Mapping, Sequence
 from typing import Any, Callable, Union
 
@@ -38,14 +39,43 @@ def apply_to_collection(data: Any, dtype: Union[type, tuple], function: Callable
     return data
 
 
+class TransferableDataType(ABC):
+    """
+    A custom type for data that can be moved to a torch device via `.to(...)`.
+
+    Example:
+
+        >>> isinstance(dict, TransferableDataType)
+        False
+        >>> isinstance(torch.rand(2, 3), TransferableDataType)
+        True
+        >>> class CustomObject:
+        ...     def __init__(self):
+        ...         self.x = torch.rand(2, 2)
+        ...     def to(self, device):
+        ...         self.x = self.x.to(device)
+        ...         return self
+        >>> isinstance(CustomObject(), TransferableDataType)
+        True
+    """
+
+    @classmethod
+    def __subclasshook__(cls, subclass):
+        if cls is TransferableDataType:
+            to = getattr(subclass, "to", None)
+            return callable(to)
+        return NotImplemented
+
+
 def move_data_to_device(batch: Any, device: torch.device):
     """
-    Transfers a collection of tensors to the given device.
+    Transfers a collection of data to the given device. Any object that defines a method
+    ``to(device)`` will be moved and all other objects in the collection will be left untouched.
 
     Args:
-        batch: A tensor or collection of tensors. See :func:`apply_to_collection`
-            for a list of supported collection types.
-        device: The device to which tensors should be moved
+        batch: A tensor or collection of tensors or anything that has a method `.to(...)`.
+            See :func:`apply_to_collection` for a list of supported collection types.
+        device: The device to which the data should be moved
 
     Return:
         the same collection but with all contained tensors residing on the new device.
@@ -54,6 +84,6 @@ def move_data_to_device(batch: Any, device: torch.device):
         - :meth:`torch.Tensor.to`
         - :class:`torch.device`
     """
-    def to(tensor):
-        return tensor.to(device, non_blocking=True)
-    return apply_to_collection(batch, dtype=torch.Tensor, function=to)
+    def to(data):
+        return data.to(device, non_blocking=True)
+    return apply_to_collection(batch, dtype=TransferableDataType, function=to)

tests/models/test_gpu.py

@@ -286,3 +286,15 @@ def test_single_gpu_batch_parse():
     batch = trainer.transfer_batch_to_gpu(batch, 0)
     assert batch[0].a.device.index == 0
     assert batch[0].a.type() == 'torch.cuda.FloatTensor'
+
+    # non-Tensor that has `.to()` defined
+    class CustomBatchType:
+        def __init__(self):
+            self.a = torch.rand(2, 2)
+
+        def to(self, *args, **kwargs):
+            self.a = self.a.to(*args, **kwargs)
+            return self
+
+    batch = trainer.transfer_batch_to_gpu(CustomBatchType())
+    assert batch.a.type() == 'torch.cuda.FloatTensor'