fix num_workers for Windows example (#5375)

* fix num_workers for Windows example

* chlog

* Apply suggestions from code review

Co-authored-by: Carlos Mocholí <carlossmocholi@gmail.com>

* warn

Co-authored-by: Carlos Mocholí <carlossmocholi@gmail.com>

CHANGELOG.md

@@ -62,6 +62,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed `DDPHPCAccelerator` hangs in DDP construction by calling `init_device` ([#5157](https://github.com/PyTorchLightning/pytorch-lightning/pull/5157))
 
 
+- Fixed `num_workers` for Windows example ([#5375](https://github.com/PyTorchLightning/pytorch-lightning/pull/5375))
+
+
 ## [1.1.3rc] - 2020-12-29
 
 ### Added

pl_examples/basic_examples/mnist_datamodule.py

@@ -13,6 +13,7 @@
 # limitations under the License.
 import platform
 from typing import Optional
+from warnings import warn
 
 from torch.utils.data import DataLoader, random_split
 
@@ -55,8 +56,10 @@ def __init__(
             normalize: If true applies image normalize
         """
         super().__init__(*args, **kwargs)
-        if platform.system() == "Windows":
-            # see: https://stackoverflow.com/a/59680818/4521646
+        if num_workers and platform.system() == "Windows":
+            # see: https://stackoverflow.com/a/59680818
+            warn(f"You have requested num_workers={num_workers} on Windows,"
+                 " but currently recommended is 0, so we set it for you")
             num_workers = 0
 
         self.dims = (1, 28, 28)