[Train] Unify configurations between LightningConfigBuilder.checkpointing() and AIR CheckpointConfig.

[woshiyyya]

Description

Specifying the same checkpoint configuration twice in two places is confusing. The user don't know which config actually controls the checkpointing logic.

The proposed solution is:

AIR CheckpointConfig	PTL checkpoint config
✅	❌	no-op, by default PTL only saves the latest checkpoint
❌	✅	create a matched AIR CheckpointConfig for users, otherwise AIR saves all checkpoints which takes tons of disk storage
✅	✅	Throw a warning if the metric doesn't match. AIR and PTL can monitor different metrics

Use case

No response