Lightly CLI not working for pytorch lightning versions < 1.5

[IgorSusmelj]

As we learned from #905 (review) the earlier pytorch lightning versions (<1.5) are not compatible with the lightly cli for training SSL models.

Let's discuss here how we want to resolve that:

• we can make an if/else statement to ensure compatibility with earlier versions. This is low effort but it's a short/mid-term solution. Newer changes of pytorch lightning might break again
• we can drop the dependency of pytorch lightning altogether (it's only used in the cli). That would be medium effort.
• we just leave it as it is knowing that older versions might not be fully compatible