Dataset Input/Output

The parameters that are used to correctly define the data inputs are the three dataset variables train, dev and eval, as well as the parameter extern_data to define the data shapes. The dataset variables are set to a dictionary structure, where the key “class” defines which class implementation to load, and the other entries are passed as parameters to the constructor of the respective dataset implementation. A list of the available datasets can be found here.

A very simple example would be:

train = {'class': 'HDFDataset', 'files': ['path/to/training_data.hdf']}

Most datasets follow the convention that the input data is sequential and has the label “data”, and the target data is sparse and has the label “classes”. In the case of the hdf file it could be that the input data are 100-dimensional MFCCs and the target data are 5,000 word classes.

The parameter extern_data can be used to give an explicit definition of the shapes. All constructor parameters to returnn.tensor.Tensor can be provided as dictionary for each data stream.

For the above example, extern_data could be defined as:

extern_data = {
  'data': {'dim': 100, 'shape': (None, 100)},
  'classes': {'dim': 5000, 'shape': (None,), 'sparse': True}

The None in the “shape” parameter tuple defines that the axis has a dynamic shape. For sequence tasks there is usually only one dynamic axis, which is specified to be the time axis. In the case of multiple dynamic axes or spatial axes it is helpful to define the time axis explicitely. For the example case of two dynamic axes, the time axis could be set to be the first axis:

extern_data = {
  'data': {'dim': 100, 'shape': (None, None, 100), 'time_dim_axis': 1},

Note that while the “shape” parameter tuple is always defined without the batch axis, the axis labels for the time, feature or the batch axis itself are counted including the batch axis. This means that “time_dim_axis: 1” corresponds to the first None of the “shape” tuple. For the general case (non-sparse data), only dim and shape are required, the other parameters are optional.

Using Layer Outputs as Data

In case you want to specify data by using layers, it is possible to add register_as_extern_data to the layer dictionary. The provided string is the key to access the data. It is not required to also add the key manually to the extern_data dictionary.

Using Multiple Data Inputs

For cases where a single dataset is not sufficient, it is possible to combine multiple datasets by using the MetaDataset.MetaDataset. Details on how to use the MetaDataset can be found here.

Synchronizing Dynamic Axes

In the case that there are multiple data streams that have exactly the same length, RETURNN does not automatically match those axis while broadcasting. The dynamic axes of different datastreams can be synchronized by using returnn.tensor.DimensionTag.

dynamic_time_dimension = DimensionTag(name="dynamic_time")

extern_data = {
  'data1': {'dim': 100, 'shape': (None, 100), 'time_dim_axis': 1, 'same_time_dim_as': {'T': dynamic_time_dimension}},
  'data2': {'dim': 10, 'shape': (None, 10), 'time_dim_axis': 1, 'same_time_dim_as': {'T': dynamic_time_dimension}},

The parameter “same_time_dims_as” takes a dictionary with axes indices or axes labels (see Managing Axes) as key and the DimensionTag as value. For the above example, there is no difference in using ‘T’ or 1 as key.

In case you want to synchronize the dynamic axes of layer outputs, a “reinterpret_data” layer (ReinterpretDataLayer) can be used. It offers the parameter size_base to copy the size_placeholder of another layer.