Use symbolic link for saving best/last checkpoints

[ZeroRin]

🚀 Feature Request

When preparing checkpoint_best.pt and checkpoint_last.pt, create symbolic link instead of making exact copy of the checkpoint

Motivation

I'm running on a machine that somehow has bad io performance, writing the same file 3 times seems extremely inefficient.

Seems that there is plan to implement a asynchronous copying but i believe a symlink is much more efficient https://github.com/facebookresearch/fairseq/blob/bedb259bf34a9fc22073c13a1cee23192fa70ef3/fairseq/checkpoint_utils.py#L128-L141

Pitch

Alternatives

Additional context