Launch options for Lightning Lite

[awaelchli]

What does this PR do?

Subclassing and overriding LightningLite.run() is no longer required. You can create and use the LightningLite object directly:

import torch.distributed
from lightning_lite import LightningLite

lite = LightningLite()
print("launched", lite.global_rank)
assert torch.distributed.is_initialized()
lite.barrier()
print("end")

To launch the script, a special CLI needs to be used:

python -m lightning_lite.cli example.py --accelerator cuda --devices 2 --precision bf16

The command above will eventually become lightning run ... in the future.

Alternatively, one can launch programmatically:

lite = LightningLite(accelerator="cuda", devices=2)
lite.launch()
...

In Jupyter notebooks (Colab, Kaggle etc.) you would launch like this:

# Notebook cell 1
def training_function(lite):
    lite.setup(model) 
    ...

# Notebook cell 2
lite = LightningLite(accelerator="cuda", devices=2)
lite.launch(training_function)

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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda @carmocca @justusschock @awaelchli

[tchaton]

@awaelchli Looks great and I really like lightning run lite ... approach.