Fix resuming checkpoint path bug

[larrylawl]

Minor fix to resuming checkpoint path . Currently, resuming takes the last path of the sorted list. However, the sorted list may not return a list sorted in ascending iter_num. For example,

from pathlib import Path
out_dir = Path("out") / "openwebtext"
paths = [out_dir / f"iter-{iter_num}-ckpt.pth" for iter_num in range(0, 1100000, 40000)]
print(sorted(paths))

returns the following. Note that the list isn't sorted (i.e. iter-1080000 should be the last, but it isn't). Thus resuming will not take the latest checkpoint in this case.

[PosixPath('out/openwebtext/iter-0-ckpt.pth'), PosixPath('out/openwebtext/iter-1000000-ckpt.pth'), PosixPath('out/openwebtext/iter-1040000-ckpt.pth'), PosixPath('out/openwebtext/iter-1080000-ckpt.pth'), PosixPath('out/openwebtext/iter-120000-ckpt.pth'), PosixPath('out/openwebtext/iter-160000-ckpt.pth'), PosixPath('out/openwebtext/iter-200000-ckpt.pth'), PosixPath('out/openwebtext/iter-240000-ckpt.pth'), PosixPath('out/openwebtext/iter-280000-ckpt.pth'), PosixPath('out/openwebtext/iter-320000-ckpt.pth'), PosixPath('out/openwebtext/iter-360000-ckpt.pth'), PosixPath('out/openwebtext/iter-40000-ckpt.pth'), PosixPath('out/openwebtext/iter-400000-ckpt.pth'), PosixPath('out/openwebtext/iter-440000-ckpt.pth'), PosixPath('out/openwebtext/iter-480000-ckpt.pth'), PosixPath('out/openwebtext/iter-520000-ckpt.pth'), PosixPath('out/openwebtext/iter-560000-ckpt.pth'), PosixPath('out/openwebtext/iter-600000-ckpt.pth'), PosixPath('out/openwebtext/iter-640000-ckpt.pth'), PosixPath('out/openwebtext/iter-680000-ckpt.pth'), PosixPath('out/openwebtext/iter-720000-ckpt.pth'), PosixPath('out/openwebtext/iter-760000-ckpt.pth'), PosixPath('out/openwebtext/iter-80000-ckpt.pth'), PosixPath('out/openwebtext/iter-800000-ckpt.pth'), PosixPath('out/openwebtext/iter-840000-ckpt.pth'), PosixPath('out/openwebtext/iter-880000-ckpt.pth'), PosixPath('out/openwebtext/iter-920000-ckpt.pth'), PosixPath('out/openwebtext/iter-960000-ckpt.pth')]

[larrylawl]

Hi, can we merge this PR in? Thank you!