Usage Guide

TorchFort provides API functions to support two common Deep Learning paradigms: supervised learning and reinforcement learning. Due to the differences in structure between these approaches, the API is organized largely into separate sets of functions designed to facilitate each approach.

Supervised Learning

Supervised learning approaches are by far the most common within Deep Learning. TorchFort provides functions to create/load/save models and run supervised training and inference on models, all directly within a C/C++/Fortran program. For a complete list of functions available, see Supervised Learning for C/C++ or Supervised Learning for Fortran bindings.

Here, we will provide a high-level guide to using TorchFort for a supervised learning problem. For a more detailed usage example, we refer to the simulation example provided in this repository in the examples/fortran directory.

Creating a model

First, create a model instance.

FortranC++

istat = torchfort_create_model(model_name, configfile)

This function instantiates a model, associating a string model_name with a model instance created using a YAML configuration file located at path configfile. See TorchFort Configuration Files for details on the YAML configuration file format and options.

Run a Training Step

Run a training step on input and label data generated from your program.

FortranC++

istat = torchfort_train(model_name, input, label, loss, stream)

This function passes the input and label data to the model instance associated with model_name and runs a training iteration, updating the model parameters as necessary. The training loss is returned by reference to the loss variable. In the Fortran API, input and label are multi-dimensional Fortran arrays, with dimension/shape/datatype information automatically inferred in the interface.

Run Inference to generate predicted output

After training the model on one or more samples, we can run an inference to generate a predicted output.

FortranC++

istat = torchfort_inference(model_name, input, output, stream)

This function passes the input data to the model instance associated with model_name and runs an inference, returning the predicted output from the model to output. In the Fortran API, input and output are multi-dimensional Fortran arrays, with dimension/shape/datatype information automatically inferred in the interface.

Checkpoint/Restart

The complete training state (current model parameters, optimizer state, learning rate scheduler progress) can be written to a checkpoint directory at any point during training.

FortranC++

istat = torchfort_save_checkpoint(model_name, directory_name)

This function will write checkpoint data for the model instance associated with model_name to the directory provided by directory_name. The directory will contain several subdirectories and files containing required information for restart.

To load a checkpoint into a created model instance, run the following:

FortranC++

istat = torchfort_load_checkpoint(model_name, directory_name, step_train, step_inference)

This function will load the checkpoint data from the directory directory_name into the model instance associated with model_name. step_train and step_inference are the checkpointed training step and inference step respectively, returned by reference.

For inference, only the model and not a full checkpoint needs to be loaded. For this, run the following function instead:

FortranC++

istat = torchfort_load_model(model_name, model_file)

This function will load the model data from the file model_file into the model instance associated with model_name. The model file can be one generated using the torchfort_save_model function or one found within a saved checkpoint directory, found at <checkpoint directory name>/model.pt.

Reinforcement Learning

Most modern reinforcement learning (RL) algorithms utilize different neural networks for policy and value functions and often require keeping track of multiple copies for each of the models (e.g., the DDPG or TD3 algorithms). Furthermore, the training algorithm causes those networks to interact in a non-trivial way. Additionally, off-policy methods require keeping track of historic data stored in a replay buffers, keeping track of actions and system states and rewards. Many reinforcement learning algorithms are deterministic in nature and thus require manual injection of randomness into the training process by employing parameter or action space noise.

The practitioner who seeks to employ these methods is often not interested in implementing all these features by hand, since it would significantly increase the complexity of the wrapped simulation application. In fact, most of these features can be reused among a broad range of applications. Therefore, instead of providing access to all the individual parts, TorchFort encapsulates all these details into a structure we call an rl_system and abstracts all the bookeeping away from the user. The user only has to configure the system and then ensure that data is added to the replay buffer and training steps or action predictions are performed whenever it is necessary from the simulation code. Routines for reinforcement learning routines are prefixed with torchfort_rl_off_policy. Currently, TorchFort only provides off-policy methods as those have been proven to be most versatile and powerful for a broad range of tasks. On-policy methods may be added in the future, hence we distinguish between these two cases in the TorchFort API.

We will provide a high-level guide for users who would like to add reinforcement learning functionality to their code. We assume that the user is familiar with the basic concepts of deep and reinforcement learning and understands the possibilities and limitations of these methods. This guide is far from exhaustive and for a complete list of reinforcement learning functions see Reinforcement Learning for C/C++ or Reinforcement Learning for Fortran respectively. We also suggest reviewing the example folder where we have implemented the cartpole RL problem in C++ using TorchFort.

Creating an RL system

To start, a TorchFort rl system has to be initialized with the call:

FortranC++

istat = torchfort_rl_off_policy_create_system(system_name, configfile)

where system_name is a name which used by TorchFort to identify the system created using YAML configuration file configfile. See TorchFort Configuration Files for details on the YAML configuration file format and options.

Replay Buffer Management

The user application (usually called environment in the RL context) will generate states and rewards based on actions suggested by the policy function or some other mechanism. For off-policy methods, this information needs to be passed to the replay buffer from which the training process will sample. This is performed via:

FortranC++

istat = torchfort_rl_off_policy_update_replay_buffer(system_name, state_old, action, state_new, reward, terminal, stream)

state_old is an array describing the old environment state to which action is applied, resulting in a new environment state state_new and a corresponding scalar reward. The variable terminal is a flag which specifies whether the end of an episode is reached. In the Fortran API, the states and actions are multi-dimensional Fortran arrays with dimension/shape/datatype automatically inferred in the interface. In the C++ API, all arrays are void pointers and the state and action dimensions and shapes have to be passed explicitly.

Note

The update replay buffer functions expect a single tuple containing single samples and hence no batch dimension should be included.

Warning

It is important to mention that this function should be called in causal order on the state tuples, i.e., the data inserted into the replay buffer should contain subsequent steps of the environment. In case of n-step rollouts, the sampling logic assumes that the list of tuples are ordered causally and different trajectories are separated by a terminal flag set to true for the last step in trajectory. Any non-causal ordering would likely yield sub-optimal training performance.

Before training can start, the replay buffer needs to contain a minimum number of state-action tuples. The readiness can be queried with:

FortranC++

istat = torchfort_rl_off_policy_is_ready(system_name, ready)

Generating Action Predictions

TorchFort provides the following two functions to generate action predictions from the policy network infrastructure:

FortranC++

istat = torchfort_rl_off_policy_predict(system_name, state, action, stream)

istat = torchfort_rl_off_policy_predict_explore(system_name, state, action, stream)

Both functions predict an action based on a state. The first variant generates a deterministic prediction from the target network (for algorithms which use target networks, i.e., a shadow network which gets updated less often than the active networks the regular weight updates are applied to). The second variant generates a prediction using the active network and also adds noise as specified in the configuration file.

Note

The prediction methods are inference methods and thus expect a batch of data. Therefore, the state and action arrays need to include a batch dimension.

Training Step

Once the system is ready, a training step (forward, backward, optimizer step, learning rate decay) can be performed by calling:

FortranC++

istat = torchfort_rl_off_policy_train_step(system_name, p_loss, q_loss)

This function will retrieve a single batch from the replay buffer and perform a training step, populating the variables p_loss, q_loss by reference.

Note

If the RL algorithm uses a policy lag bigger than zero, for some steps only the value function networks are updated. In this case, p_loss is not computed and will be equal to zero.

Checkpoint/Restart

At any time during or after training, a checkpoint of the full system can be stored using:

FortranC++

istat = torchfort_rl_off_policy_save_checkpoint(system_name, directory_name)

This will save everything from the RL system with name system_name into the directory with name directory_name. This checkpoint includes all models, i.e. all value and policy functions, active and target, multiple critics, etc. It will also save all optimizer and learning rate scheduler states. Additionally, the function will also save the replay buffer data and additional information such as episode number. This is required for being able to restore the full state of the RL system via:

FortranC++

istat = torchfort_rl_off_policy_load_checkpoint(system_name, directory_name)

This function is only required if RL training from the checkpoint should be resumed. In an inference setting, where only the policy should be run, the method:

FortranC++

istat = torchfort_load_model(model_name, policy_model_file);

can be used instead. The model instance should be created beforehand using the methods described in the Supervised Learning section.