build: fetch PyTorch version using downloaded WHL file (llvm#1632)

Until recently, the metadata file in the torchvision package included
the nightly version of the torch package, but since that is no longer
the case, our RollPyTorch workflow is broken.

As a workaround, this patch uses the `pip download` command's ability to
fetch the dependent torch package for the specified version of
torchvision, before peeking into the WHL file for the torch package to
determine the release version and the commit hash.

.github/workflows/RollPyTorch.yml

@@ -37,16 +37,13 @@ jobs:
         echo "Found torchvision release ${VISION_RELEASE}"
 
         # Fetch the whl file associated with the nightly torchvision release
-        rm -f torchvision-"${VISION_RELEASE}"*.whl
+        rm -f torch*.whl
         python -m pip download -f https://download.pytorch.org/whl/nightly/cpu/torch_nightly.html --pre "torchvision==${VISION_RELEASE}"
 
-        PT_RELEASE=$(unzip -p torchvision-"${VISION_RELEASE}"*.whl 'torchvision-*/METADATA' | grep "^Requires-Dist: torch" | tr -d "()=" | awk '{ print $3 }' | sed 's/\([^+]*\).*/\1/')
+        # Downloading the torchvision WHL also downloads the PyTorch WHL file
+        # Read the version from the downloaded whl file without extracting it
+        PT_RELEASE=$(unzip -p torch-*.whl 'torch-*/METADATA' | grep "^Version:" | awk '{ print $2 }' | sed 's/\([^+]*\).*/\1/')
         echo "Found torch release ${PT_RELEASE}"
-
-        # Fetch the whl file associated with the nightly release
-        rm -f torch-"${PT_RELEASE}"*.whl
-        python -m pip download -f https://download.pytorch.org/whl/nightly/cpu/torch_nightly.html --pre "torch==${PT_RELEASE}"
-
         printf -- "-f https://download.pytorch.org/whl/nightly/cpu/torch_nightly.html\n--pre\ntorch==%s\ntorchvision==%s\n" "${PT_RELEASE}" "${VISION_RELEASE}" > pytorch-requirements.txt
 
         # Read the commit hash from the downloaded whl file without extracting it