Operations on dimension tags

The user can manually specify dimension tags (DimensionTag) in the config and assign them to extern_data via same_dim_tags_as.

Otherwise they will be automatically created for the extern data keys.

Some layers will create new dimension tags in the output, such as:

• LinearLayer, ConvLayer, RecLayer and others can transform a static dim (feature dim) into another static dim (new feature dim, specified via n_out), usually via linear transformation
• ConvLayer, PoolLayer, ResizeLayer and others can transform spatial axes (via striding, or valid padding, etc). See layers which introduce new dynamic sequence lengths.
• ConstantLayer, VariableLayer, RandIntLayer and others can create arbitrary new dimensions and dimension tags. Dimension tags can be explicitly created by the user. For dynamic dims, explicit dim tags must be used.
• ReinterpretDataLayer can assign explicit new dim tags.
• CumConcatLayer assigns a new dim tag

Some layers reduce or remove axes, such as:

• ReduceLayer
• DotLayer