Add helper to download preprocessed aggregated nextstrain dataset (#2978

)

* Add code to download nextstrain dataset

* Add docs

* Address review comment

docs/source/contrib.examples.rst

@@ -20,6 +20,14 @@ BART Ridership
     :show-inheritance:
     :member-order: bysource
 
+Nextstrain SARS-CoV-2 counts
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+.. automodule:: pyro.contrib.examples.nextstrain
+    :members:
+    :undoc-members:
+    :show-inheritance:
+    :member-order: bysource
+
 Utilities
 ---------
 .. automodule:: pyro.contrib.examples.util

pyro/contrib/examples/nextstrain.py

@@ -0,0 +1,44 @@
+# Copyright Contributors to the Pyro project.
+# SPDX-License-Identifier: Apache-2.0
+
+import logging
+import os
+import subprocess
+import urllib
+
+import torch
+
+from .util import _mkdir_p, get_data_directory
+
+DATA = get_data_directory(__file__)
+URL = "https://github.com/pyro-ppl/datasets/raw/master/nextstrain.data.pt.gz"
+
+
+def load_nextstrain_counts(map_location=None) -> dict:
+    """
+    Loads a SARS-CoV-2 dataset.
+
+    The original dataset is a preprocessed intermediate ``metadata.tsv.gz`` available via
+    `nextstrain <https://docs.nextstrain.org/projects/ncov/en/latest/reference/remote_inputs.html>`_.
+    The ``metadata.tsv.gz`` file was then aggregated to
+    (month,location,lineage) and (lineage,mutation) bins by the Broad Institute's
+    `preprocessing script <https://github.com/broadinstitute/pyro-cov/blob/master/scripts/preprocess_nextstrain.py>`_.
+    """
+    # Download the gzipped file.
+    _mkdir_p(DATA)
+    basename = URL.split("/")[-1]
+    gz_filename = os.path.join(DATA, basename)
+    if not os.path.exists(gz_filename):
+        logging.debug(f"downloading {URL}")
+        urllib.request.urlretrieve(URL, gz_filename)
+
+    # Decompress the file.
+    filename = gz_filename.replace(".gz", "")
+    if not os.path.exists(filename):
+        logging.debug(f"unzipping {gz_filename}")
+        subprocess.check_call(["gunzip", "-k", gz_filename])
+
+    # Load tensors to the default location.
+    if map_location is None:
+        map_location = torch.tensor(0.0).device
+    return torch.load(filename, map_location=map_location)