[bugfix] TPU + all_gather + SingleTPU shouldn't call xm.all_gather (#6296)

* resolve an issue with TPU

* update

* add changelog

CHANGELOG.md

@@ -95,6 +95,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed duplicate logs appearing in console when using the python logging module ([#5509](https://github.com/PyTorchLightning/pytorch-lightning/pull/5509), [#6275](https://github.com/PyTorchLightning/pytorch-lightning/pull/6275))
 
 
+- Fixed `SingleTPU` calling `all_gather` ([#6296](https://github.com/PyTorchLightning/pytorch-lightning/pull/6296))
+
+
 - Fixed error thrown when using valid distributed mode in multi node ([#6297](https://github.com/PyTorchLightning/pytorch-lightning/pull/6297)
 
 

pytorch_lightning/accelerators/tpu.py

@@ -44,4 +44,7 @@ class TPUAccelerator(Accelerator):
         Return:
             A tensor of shape (world_size, batch, ...)
         """
+        # todo: Add support for backward with all_gather
+        if torch.distributed.is_initialized():
             return xm.all_gather(tensor, group=group, sync_grads=sync_grads)
+        return tensor