Allow for no explicit pads in Conv layer in channel last conversion

[jmitrevs]

It seems like pads should default to zero in a conv layer if it's not set and auto_pads is either not set or set to 'NOTSET'. It seems like we currently don't support auto_pads during the channel last conversion--should we? This PR fixes the case when pads is not set, treating it as if it's all zeros.