(app) Introduce LightningTrainingComponent

[tchaton]

What does this PR do?

This PR introduces a LightningTrainingComponent which provides multi-node training in the cloud.

from lightning import LightningApp
from lightning.app.components.training import LightningTrainingComponent
from lightning_app.utilities.packaging.cloud_compute import CloudCompute

app = LightningApp(
    LightningTrainingComponent(
        "train.py",
        num_nodes=2,
        cloud_compute=CloudCompute("CPU"),
    ),
)

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

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cc @Borda