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

* resolve an issue with TPU

* update

* add changelog

Author: tchaton

CHANGELOG.md

@@ -57,6 +57,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed `trainer.test` from `best_path` hangs after calling `trainer.fit`  ([#6272](https://github.com/PyTorchLightning/pytorch-lightning/pull/6272))
 
 
+- Fixed `SingleTPU` calling `all_gather` ([#6296](https://github.com/PyTorchLightning/pytorch-lightning/pull/6296))
+
 ## [1.2.2] - 2021-03-02
 
 ### Added