Checkpointer Saves Model when it outputs NaN

[sciapponi]

When I'm training a model and gradients explode/vanish Micromind still saves it to the checkpoint folder.
Personally I'd like the framework to keep the last working version since it could be useful to identify issues about the weight computation.

[fpaissan]

Thanks, @sciapponi, for letting us know about this problem.
I'll take a look an create a PR once this is tested. I'll make sure to keep you posted here.