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Design for TensorArray

TensorArray as a new concept is borrowed from TensorFlow, it is meant to be used with dynamic iteration primitives such as while_loop and map_fn.

This concept can be used to support our new design of dynamic operations, and help to refactor some existing variant-sentence-related layers, such as RecurrentGradientMachine.

In our design for dynamic RNN, TensorArray is used to segment inputs and store states in all time steps. By providing some methods similar to a C++ array, the definition of some state-based dynamic models such as RNN could be more natural and highly flexible.

Some basic methods should be proposed as follows:

stack()

Pack the values in a TensorArray into a tensor with rank one higher than each tensor in values.

unstack(axis=0)

Unpacks the given dimension of a rank-R tensor into rank-(R-1) tensors.

concat()

Return the values in the TensorArray as a concatenated Tensor.

write(index, value, data_shared=true)

Write value into index of the TensorArray.

read(index)

Read the value at location index in the TensorArray.

size()

Return the number of values.

The RecurrentGradientMachine in Paddle serves as a flexible RNN layer; it takes variant length sequences as input, because each step of RNN could only take a tensor-represented batch of data as input, some preprocess should be taken on the inputs such as sorting the sentences by their length in descending order and cut each word and pack to new batches.

Such cut-like operations can be embedded into TensorArray as general methods called unpack and pack.

With these two methods, a variant-sentence-RNN can be implemented like

// input is the varient-length data
LodTensor sentence_input(xxx);
TensorArray ta;
Tensor indice_map;
Tensor boot_state = xxx; // to initialize rnn's first state
TensorArray::unpack(input, 1/*level*/, true/*sort_by_length*/, &ta, &indice_map);
TessorArray step_outputs;
TensorArray states;

for (int step = 0; step = ta.size(); step++) {
  auto state = states.read(step);
  // rnnstep is a function which acts like a step of RNN
  auto step_input = ta.read(step);
  auto step_output = rnnstep(step_input, state);
  step_outputs.write(step_output, true/*data_shared*/);
}

// rnn_output is the final output of an rnn
LoDTensor rnn_output = ta.pack(ta, indice_map);

the code above shows that by embedding the LoDTensor-related preprocess operations into TensorArray, the implementation of a RNN that supports varient-length sentences is far more concise than RecurrentGradientMachine because the latter mixes all the codes together, hard to read and extend.

some details are as follows.

unpack(level, sort_by_length)

Split LodTensor in some level and generate batches, if set sort_by_length, will sort by length.

Returns:

a new TensorArray, whose values are LodTensors and represents batches of data.
an int32 Tensor, which stores the map from the new batch's indices to original LoDTensor

pack(level, indices_map)

Recover the original LoD-arranged LoDTensor with the values in a TensorArray and level and indices_map.

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