Drop torch 1.6 support

[carmocca]

What does this PR do?

We have a 4 PyTorch version policy. With the release of 1.10, that means we can drop 1.6

[1.6, 1.7, 1.8, 1.9, 1.10] -> [1.7, 1.8, 1.9, 1.10]

Part of #10075

Does your PR introduce any breaking changes? If yes, please list them.

None

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[github-actions]

Build Error! No Linked Issue found. Please link an issue or mention it in the body using #<issue_id>

[justusschock]

LGTM overall.

I think whereever possible (all runs/images with python 3.8) we should still fall back to 1.7 and only for the 3.9 versions lift it to 1.7.1

[carmocca]

@justusschock we could do that but would complicate CI a bit since we would need logic to handle 1.7 and 1.7.1 separately, additionally, we would need to generate both docker images.

I think 1.7.1 is fine looking at the release notes: https://github.com/pytorch/pytorch/releases/tag/v1.7.1
and download stats per version: https://pepy.tech/project/torch?versions=1.7.0&versions=1.7.1&versions=1.10.0&versions=1.6.0

And technically this should have been done before the 1.5 release, because it supports 1.6-1.10 (5 versions)

so we should actually drop 1.7 too before the 1.6 release, as it should support 1.8-1.11 (4 versions)

edit: found a solution using wildcard in c498a3b

[akihironitta]

LGTM!

Does this mean we can also deprecate the following lines and remove it in pl v1.7, right?

https://github.com/PyTorchLightning/pytorch-lightning/blob/3a6acadd115e86f02d83a788f1978372ab6764f3/pytorch_lightning/overrides/torch_distributed.py#L13-L14

[carmocca]

Does this mean we can also deprecate the following lines and remove it in pl v1.7, right?

yes, in #10390

[Borda]

@carmocca I would rather keep it on master as we are going to merge bug fixes which will be conservatively cherry picked to release/1.5.x where we still support PT 1.6
So it may yield in extra work and debugging if the fix is touching some old PT functionality...

[carmocca]

@Borda This PR should have landed with 1.5 already to comply with the 4 torch versions window.

Still, this can be merged to master and the milestone changed if you want to land a specific fix for 1.5.0 with 1.6

[tchaton]

LGTM !