Add the metrics completion_length_max and completion_length_min

[dignfei]

…h are very useful for monitoring the training process.

What does this PR do?

Add the metrics completion_length_max and completion_length_min, which are very useful for monitoring the training process.

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[qgallouedec]

wdyt of having an histogram instead? Example: wandb.Histogram(torch.randn(20).tolist()) but with the completion lengths