Set default strategy to ddp_fork in interactive environments

[awaelchli]

What does this PR do?

Follow up to #13744 and #13405.

This PR makes it so that in Jupyter notebooks, you only need two specify

Trainer(devices=2)

instead of

Trainer(strategy="ddp_notebook", devices=2)

similar to the TPU strategy being auto-selected when accelerator="tpu".

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

No.

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Did you have fun?

I made sure I had fun coding 🙃
cc @Borda @justusschock @kaushikb11 @awaelchli @akihironitta @rohitgr7

[carmocca]

Sweet