Use the non-protected LRScheduler import

🚀 Feature

Motivation

Avoid protected imports

Pitch

https://github.com/pytorch/pytorch/issues/61232 has been merged to PyTorch which renames _LRScheduler to LRScheduler. They have kept the old class for compatibility, but we should still use the new one.

The task is to add logic like this

LRScheduler = (
    torch.optim.lr_scheduler.LRScheduler
    if _TORCH_GREATER_EQUAL_1_14 else 
    torch.optim.lr_scheduler._LRScheduler
)

to https://github.com/Lightning-AI/lightning/blob/d5003b1c07fda783f651a732c86ad48656be42c1/src/lightning_lite/utilities/types.py#L66 and places that use it

Alternatives

Keep using the protected import

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cc @borda

[qmaruf]

Hi @shenoynikhil, i am working on this one.

[carmocca]

I described the work in the top post. We want to use LRScheduler throughout the codebase by importing a small compatibility variable defined in lightning/src/lightning_lite/utilities/types.py