Training a PyTorch Model

This tutorial shows how to train a Logistic Regression model in PyTorch using the Census API’s experimental.ml.ExperimentDataPipe class. This is intended only to demonstrate the use of the ExperimentDataPipe, and not as an example of how to train a biologically useful model.

This tutorial assumes a basic familiarity with PyTorch and the Census API. See the Querying and fetching the single-cell data and cell/gene metadata notebook tutorial for a quick primer on Census API usage.

Contents

Open the Census

First, obtain a handle to the Census data, in the usual manner:

[25]:
import cellxgene_census

census = cellxgene_census.open_soma()
The "stable" release is currently 2023-07-25. Specify 'census_version="2023-07-25"' in future calls to open_soma() to ensure data consistency.

Create an ExperimentDataPipe

To train a model in PyTorch using this census data object, first instantiate an ExperimentDataPipe as follows:

[26]:
import cellxgene_census.experimental.ml as census_ml
import tiledbsoma as soma

experiment = census["census_data"]["homo_sapiens"]

experiment_datapipe = census_ml.ExperimentDataPipe(
    experiment,
    measurement_name="RNA",
    X_name="raw",
    obs_query=soma.AxisQuery(value_filter="tissue_general == 'tongue' and is_primary_data == True"),
    obs_column_names=["cell_type"],
    batch_size=128,
    shuffle=True,
    soma_chunk_size=10_000,
)

ExperimentDataPipe class explained

This class provides an implementation of PyTorch’s DataPipe interface, which defines a common mechanism for wrapping and accessing training data from any underlying source. The ExperimentDataPipe class encapsulates the details of querying and retrieving Census data from a single SOMA Experiment and returning it to the caller as PyTorch Tensors. Most importantly, it retrieves the data lazily from the Census in batches, avoiding having to load the entire training dataset into memory at once. (Note: PyTorch also provides DataSet as a legacy interface for wrapping and accessing training data sources, but a DataPipe can be used interchangeably.)

ExperimentDataPipe parameters explained

The constructor only requires a single parameter, experiment, which is a soma.Experiment containing the data of the organism to be used for training.

To retrieve a subset of the Experiment’s data, along either the obs or var axes, you may specify query filters via the obs_query and var_query parameters, which are both soma.AxisQuery objects.

The values for the prediction label(s) that you intend to use for training are specified via the obs_column_names array.

The batch_size allows you to specify the number of obs rows (cells) to be returned by each return PyTorch tensor. You may exclude this parameter if you want single rows (batch_size=1).

The shuffle flag allows you to randomize the ordering of the training data for each training epoch. Note: * You should use this flag instead of the DataLoader shuffle flag, as DataLoader does not support shuffling when used with an IterDataPipe dataset. * PyTorch’s TorchData library provides a Shuffler DataPipe, which is alternate mechanism one can use to perform shuffling of an IterDataPipe. However, the Shuffler will not “globally” randomize the training data, as it only “locally” randomizes the ordering of the training data within fixed-size “windows”. Due to the layout of Census data, a given “window” of Census data may be highly homogeneous in terms of its obs axis attribute values, and so this shuffling strategy may not provide sufficient randomization for certain types of models.

The soma_chunk_size sets the number of rows of data that are retrieved from the Census and held in memory at a given time. This controls the maximum memory usage of the ExperimentDataPipe. Smaller values will require less memory but will also result in lower read performance. If you are running out of memory when training a model, try reducing this value. The default is set to retrieve ~1GB of data per chunk, which takes into account how many var (gene) columns are being requested. This parameter also affects the granularity of the “global” shuffling step when shuffle=True (see shuffle parameter API docs for details).

You can inspect the shape of the full dataset, without causing the full dataset to be loaded:

[27]:
experiment_datapipe.shape
[27]:
(15020, 60664)

Split the dataset

You may split the overall dataset into the typical training, validation, and test sets by using the PyTorch RandomSplitter DataPipe. Using PyTorch’s functional form for chaining DataPipes, this is done as follows:

[28]:
train_datapipe, test_datapipe = experiment_datapipe.random_split(weights={"train": 0.8, "test": 0.2}, seed=1)

Create the DataLoader

With the full set of DataPipe operations chained together, we can now instantiate a PyTorch DataLoader on the training data.

[29]:
experiment_dataloader = census_ml.experiment_dataloader(train_datapipe)

Alternately, you can instantiate a DataLoader object directly via its constructor. However, many of the parameters are not usable with iterable-style DataPipes, which is the case for ExperimentDataPipe. In particular, the shuffle, batch_size, sampler, batch_sampler, collate_fn parameters should not be specified. Using experiment_dataloader helps enforce correct usage.

Define the model

With the training data retrieval code now in place, we can move on to defining a simple logistic regression model, using PyTorch’s torch.nn.Linear class:

[30]:
import torch


class LogisticRegression(torch.nn.Module):
    def __init__(self, input_dim, output_dim):
        super(LogisticRegression, self).__init__()
        self.linear = torch.nn.Linear(input_dim, output_dim)

    def forward(self, x):
        outputs = torch.sigmoid(self.linear(x))
        return outputs

Next, we define a function to train the model for a single epoch:

[31]:
def train_epoch(model, train_dataloader, loss_fn, optimizer, device):
    model.train()
    train_loss = 0
    train_correct = 0
    train_total = 0

    for batch in train_dataloader:
        optimizer.zero_grad()
        X_batch, y_batch = batch

        X_batch = X_batch.float().to(device)

        # Perform prediction
        outputs = model(X_batch)

        # Determine the predicted label
        probabilities = torch.nn.functional.softmax(outputs, 1)
        predictions = torch.argmax(probabilities, axis=1)

        # Compute the loss and perform back propagation

        # Exclude the cell_type labels, which are in the second column
        y_batch = y_batch[:, 1]
        y_batch = y_batch.to(device)

        train_correct += (predictions == y_batch).sum().item()
        train_total += len(predictions)

        loss = loss_fn(outputs, y_batch.long())
        train_loss += loss.item()
        loss.backward()
        optimizer.step()

    train_loss /= train_total
    train_accuracy = train_correct / train_total
    return train_loss, train_accuracy

Note the line, X_batch, y_batch = batch. Since the train_dataloader was configured with batch_size=16, these variables will hold tensors of rank 2. The X_batch tensor will appear, for example, as:

tensor([[0., 0., 0.,  ..., 1., 0., 0.],
        [0., 0., 2.,  ..., 0., 3., 0.],
        [0., 0., 0.,  ..., 0., 0., 0.],
        ...,
        [0., 0., 0.,  ..., 0., 0., 0.],
        [0., 1., 0.,  ..., 0., 0., 0.],
        [0., 0., 0.,  ..., 0., 0., 8.]])

For batch_size=1, the tensors will be of rank 1. The X_batch tensor will appear, for example, as:

tensor([0., 0., 0.,  ..., 1., 0., 0.])

Secondly, note the line, y_batch = y_batch[:, 1]. This line is extracting the user-specified obs cell_type training labels from the second column of the y_batch rank 2 Tensor. For example, this would take a y_batch tensor that looks like:

tensor([[42496620,        1],
        [42496621,        1],
        [42496622,        3],
        ...,
        [42496633,        2],
        [42496634,        1],
        [42496635,        4]], dtype=torch.int32)

and return:

tensor([1, 1, 3, ..., 2, 1, 4])

Note that cell type values are integer-encoded values, which can be decoded using experiment_datapipe.obs_encoders (more on this below).

Train the model

Finally, we are ready to train the model. Here we instantiate the model, a loss function, and an optimization method and then iterate through the desired number of training epochs. Note how the train_dataloader is passed into train_epoch, where for each epoch it will provide a new iterator through the training dataset.

[32]:
device = torch.device("cuda") if torch.cuda.is_available() else torch.device("cpu")

# The size of the input dimension is the number of genes
input_dim = experiment_datapipe.shape[1]

# The size of the output dimension is the number of distinct cell_type values
cell_type_encoder = experiment_datapipe.obs_encoders["cell_type"]
output_dim = len(cell_type_encoder.classes_)

model = LogisticRegression(input_dim, output_dim).to(device)
loss_fn = torch.nn.CrossEntropyLoss()
optimizer = torch.optim.Adam(model.parameters(), lr=1e-05)

for epoch in range(10):
    train_loss, train_accuracy = train_epoch(model, experiment_dataloader, loss_fn, optimizer, device)
    print(f"Epoch {epoch + 1}: Train Loss: {train_loss:.7f} Accuracy {train_accuracy:.4f}")
Epoch 1: Train Loss: 0.0167253 Accuracy 0.4856
Epoch 2: Train Loss: 0.0156710 Accuracy 0.4943
Epoch 3: Train Loss: 0.0149408 Accuracy 0.4813
Epoch 4: Train Loss: 0.0144469 Accuracy 0.5040
Epoch 5: Train Loss: 0.0141749 Accuracy 0.5669
Epoch 6: Train Loss: 0.0139776 Accuracy 0.6672
Epoch 7: Train Loss: 0.0138565 Accuracy 0.7920
Epoch 8: Train Loss: 0.0138094 Accuracy 0.8088
Epoch 9: Train Loss: 0.0136689 Accuracy 0.8757
Epoch 10: Train Loss: 0.0136101 Accuracy 0.8923

Make predictions with the model

To make predictions with the model, we first create a new DataLoader using the test_datapipe, which provides the “test” split of the original dataset. For this example, we will only make predictions on a single batch of data from the test split.

[33]:
experiment_dataloader = census_ml.experiment_dataloader(test_datapipe)
X_batch, y_batch = next(iter(experiment_dataloader))

Next, we invoke the model on the X_batch input data and extract the predictions:

[34]:
model.eval()

model.to(device)
outputs = model(X_batch.to(device))

probabilities = torch.nn.functional.softmax(outputs, 1)
predictions = torch.argmax(probabilities, axis=1)

display(predictions)
tensor([ 1, 11,  1,  1,  5,  1,  1,  1,  1,  5,  1,  5,  1,  5,  5,  8,  1,  1,
         7,  1,  5,  5,  8,  5,  5,  1,  1,  1,  1,  8,  9,  1,  1,  8,  1,  1,
         1, 11,  5,  1,  8,  5,  5,  1,  5,  1,  5,  5,  1,  5,  9,  8,  1,  1,
         1,  5,  5,  5,  1,  5,  1,  5,  1,  1,  5,  8,  1,  1,  1,  1,  7,  1,
         5,  1,  1,  5,  5,  1,  1,  8,  5,  5,  8,  1,  1,  1,  5,  5,  5,  1,
         5,  1,  5,  5,  1,  1,  5,  1,  5,  1,  1,  1,  5,  1,  1,  1,  9,  5,
         1,  1,  7,  1,  1,  1,  1,  8,  1,  1,  5,  5,  1,  5,  1,  1,  1,  5,
         8,  1])

The predictions are returned as the encoded values of cell_type label. To recover the original cell type labels as strings, we decode using the encoders from experiment_datapipe.obs_encoders.

At inference time, if the model inputs are not obtained via an ExperimentDataPipe, one could pickle the encoder at training time and save it along with the model. Then, at inference time it can be unpickled and used as shown below.

[35]:
cell_type_encoder = experiment_datapipe.obs_encoders["cell_type"]

predicted_cell_types = cell_type_encoder.inverse_transform(predictions.cpu())

display(predicted_cell_types)
array(['basal cell', 'vein endothelial cell', 'basal cell', 'basal cell',
       'epithelial cell', 'basal cell', 'basal cell', 'basal cell',
       'basal cell', 'epithelial cell', 'basal cell', 'epithelial cell',
       'basal cell', 'epithelial cell', 'epithelial cell', 'leukocyte',
       'basal cell', 'basal cell', 'keratinocyte', 'basal cell',
       'epithelial cell', 'epithelial cell', 'leukocyte',
       'epithelial cell', 'epithelial cell', 'basal cell', 'basal cell',
       'basal cell', 'basal cell', 'leukocyte', 'pericyte', 'basal cell',
       'basal cell', 'leukocyte', 'basal cell', 'basal cell',
       'basal cell', 'vein endothelial cell', 'epithelial cell',
       'basal cell', 'leukocyte', 'epithelial cell', 'epithelial cell',
       'basal cell', 'epithelial cell', 'basal cell', 'epithelial cell',
       'epithelial cell', 'basal cell', 'epithelial cell', 'pericyte',
       'leukocyte', 'basal cell', 'basal cell', 'basal cell',
       'epithelial cell', 'epithelial cell', 'epithelial cell',
       'basal cell', 'epithelial cell', 'basal cell', 'epithelial cell',
       'basal cell', 'basal cell', 'epithelial cell', 'leukocyte',
       'basal cell', 'basal cell', 'basal cell', 'basal cell',
       'keratinocyte', 'basal cell', 'epithelial cell', 'basal cell',
       'basal cell', 'epithelial cell', 'epithelial cell', 'basal cell',
       'basal cell', 'leukocyte', 'epithelial cell', 'epithelial cell',
       'leukocyte', 'basal cell', 'basal cell', 'basal cell',
       'epithelial cell', 'epithelial cell', 'epithelial cell',
       'basal cell', 'epithelial cell', 'basal cell', 'epithelial cell',
       'epithelial cell', 'basal cell', 'basal cell', 'epithelial cell',
       'basal cell', 'epithelial cell', 'basal cell', 'basal cell',
       'basal cell', 'epithelial cell', 'basal cell', 'basal cell',
       'basal cell', 'pericyte', 'epithelial cell', 'basal cell',
       'basal cell', 'keratinocyte', 'basal cell', 'basal cell',
       'basal cell', 'basal cell', 'leukocyte', 'basal cell',
       'basal cell', 'epithelial cell', 'epithelial cell', 'basal cell',
       'epithelial cell', 'basal cell', 'basal cell', 'basal cell',
       'epithelial cell', 'leukocyte', 'basal cell'], dtype=object)

Finally, we create a Pandas DataFrame to examine the predictions:

[36]:
import pandas as pd

display(
    pd.DataFrame(
        {
            "actual cell type": cell_type_encoder.inverse_transform(y_batch[:, 1].numpy()),
            "predicted cell type": predicted_cell_types,
        }
    )
)
actual cell type predicted cell type
0 basal cell basal cell
1 vein endothelial cell vein endothelial cell
2 basal cell basal cell
3 basal cell basal cell
4 epithelial cell epithelial cell
... ... ...
123 basal cell basal cell
124 basal cell basal cell
125 epithelial cell epithelial cell
126 leukocyte leukocyte
127 basal cell basal cell

128 rows × 2 columns