Fabric run CLI cannot launch python module

[jhliu17]

Description & Motivation

In Fabric run CLI, the script argument always assumes the input is an existed file path. However, users might need to launch a python module.

pytorch-lightning/src/lightning/fabric/cli.py

Line 63 in 08266a9

type=click.Path(exists=True),

For example, the CLI expects

fabric run src/run.py \
    --strategy=ddp \
    --devices=2 \
    --accelerator=cuda

However, this would fail

fabric run \
    --strategy=ddp \
    --devices=2 \
    --accelerator=cuda \
    -m src.run

In huggingface accelearte, they provide an extra argument for definiing an module
https://github.com/huggingface/accelerate/blob/4b6be8991059f39a8df8893333d11c54bc51fc60/src/accelerate/commands/launch.py#L358

Pitch

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Alternatives

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Additional context

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cc @lantiga @Borda