Create a link for the best checkpoint

[shenmishajing]

Description & Motivation

As illustrated in #19294 , since we already implement the link of last.ckpt, how about adding a new feature on best.ckpt? Although we support saving the topk checkpoints already, there is no consistent way to reference the top1 checkpoint.

For example, in the above figure, I have to check the files under the folder work_dirs/medclip_resnet50_iu_xray_R2GenGPT_30x/2024.01.16_17.44.40.927262/checkpoints to determine the file name epoch:5-val_loss:3.264.ckpt to reference the best checkpoint if I have not created the best.ckpt checkpoint. It will be more convenient if we implement the hard link of best.ckpt also. It can be implemented with the similar code as the last.ckpt part.

Pitch

Create a link for the best checkpoint when it is updated.

Alternatives

No response

Additional context

No response

cc @Borda @carmocca @awaelchli