Fix step size extraction for checkpoint name

[lzhang10]

This fixes a bug introduced in 47ced4e

Fix step size extraction for checkpoint name with - in it such as avg-model.ckpt-1234

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[lzhang10]

I signed it.

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[lzhang10]

I signed it!

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CLAs look good, thanks!

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[afrozenator]

Thanks a lot for finding and fixing @lzhang10 !!