all_gather raises NotImplementedError when no Accelerator defined in Trainer
🐛 Bug

When no Accelerator is defined in Trainer, all_gather function in LightningModule raises NotImplementedError.
Please reproduce using the BoringModel and post here

https://colab.research.google.com/drive/1VPEIaQ-aN5KVA70VtvGk24AVkTYPvMoY?usp=sharing
To Reproduce

Expected behavior


All gather should return a value instead.

Environment

CUDA:

GPU:

Tesla P100-PCIE-16GB


available:         True
version:           10.1


Packages:

numpy:             1.19.4
pyTorch_debug:     True
pyTorch_version:   1.7.0+cu101
pytorch-lightning: 1.1.2rc1
tqdm:              4.41.1


System:

OS:                Linux
architecture:

64bit



processor:         x86_64
python:            3.6.9
version:           #1 SMP Thu Jul 23 08:00:38 PDT 2020



Additional context