check for kaggle env variable (#1568)

* check for kaggle env variable

* added changelog

CHANGELOG.md

@@ -26,6 +26,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 - Added [Horovod](http://horovod.ai) support as a distributed backend `Trainer(distributed_backend='horovod')` ([#1529](https://github.com/PyTorchLightning/pytorch-lightning/pull/1529))
 
+- Added support for 8 core distributed training on Kaggle TPU's (https://github.com/PyTorchLightning/pytorch-lightning/pull/1568)
+
 ### Changed
 
 - Changed the default behaviour to no longer include a NaN check with each training iteration. ([#1475](https://github.com/PyTorchLightning/pytorch-lightning/pull/1475))

pytorch_lightning/trainer/trainer.py

@@ -755,7 +755,7 @@ class Trainer(
             log.info(f'training on {self.num_tpu_cores} TPU cores')
 
             #  COLAB_GPU is an env var available by default in Colab environments.
-            start_method = 'fork' if os.getenv('COLAB_GPU') else 'spawn'
+            start_method = 'fork' if os.getenv('COLAB_GPU') or os.getenv('KAGGLE_URL_BASE') else 'spawn'
 
             # track for predict
             self.model = model