[fsdp, doc] feat: Enhance FSDP checkpointing with DCP support

[dkremez]

This commit introduces support for PyTorch Distributed Checkpoint (DCP) format in FSDP checkpointing, improving performance and reliability. Key changes include:

• Updated documentation to reflect the new DCP format and its benefits.
• Added methods to check for DCP format and load DCP checkpoints in fsdp_model_merger.py.
• Enhanced FSDPCheckpointManager to handle DCP saves and loads, including support for asynchronous saving.
• Updated configuration to indicate that async_save is now supported for both FSDP and Megatron backends.

These changes streamline the checkpointing process and ensure compatibility with the latest PyTorch features.

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