[ds-inference] checkpoint loading => tqdm

[stas00]

solve 2 issues:

• less noise using tqdm progress bar
• more informative - tell users how much to wait and how many shards to load

New way:

Loading 72 checkpoints:  12%|█▎        | 9/72 [01:12<08:39,  8.25s/it]

@RezaYazdaniAminabadi, @jeffra

[stas00]

It's actually interesting to watch tqdm processing here - the first checkpoints are loaded really fast and then it starts slowing down and elongating the ETA as the IO gets saturated.

Unrelated to this PR, but perhaps we can use a different approach to loading, so that the io per shard happens only once per node and not 8 or 16 times concurrently? mmap or something similar? It'd probably make the loading much much faster. 10min to load is quite slow.

and of course we discussing making 2 additional branches with pre-sharded-per-TP-rank data, so each process loads only what it needs. But that requires a lot more work and coding to look up the right checkpoint branch.