CI: create PR for RollPyTorch updates (llvm#2106)

Currently, we run just the Linux in-tree tests when the RollPyTorch
workflow runs, but this is insufficient since WHL files for macOS or
Windows are sometimes not uploaded by PyTorch, causing the RollPyTorch
action to pass but all subsequent torch-mlir CI tests to fail because of
the broken build.

The easiest way to validate the RollPyTorch action on all platforms is
to run the standard set of tests that we run for each submitted PR, so
this patch makes the RollPyTorch action submit a PR instead of
committing the changes to the main branch directly.  The PR is assigned
to a handful of folks for review, although this can be changed in the
future.

.github/workflows/RollPyTorch.yml

@@ -97,7 +97,15 @@ jobs:
           torchvision version: ${{ env.PTVISION_RELEASE }}
           ```
 
-    - name: Push changes to main branch
+    - name: Update PyTorch Build Cache (if running on main branch)
+      if: github.ref_name == 'main'
+      id: cache-pytorch
+      uses: ashay/cache@v1
+      with:
+        path: ${{ github.workspace }}/build_tools/python_deploy/wheelhouse
+        key: ${{ runner.os }}-pytorch-${{ env.PT_HASH }}
+
+    - name: Commit changes locally
       if: env.PT_HASH_CHANGED != '0'
       run: |
         cd ${GITHUB_WORKSPACE}
@@ -106,13 +114,25 @@ jobs:
         git fetch --recurse-submodules=no
         git checkout main
         git pull origin main
+        git checkout -b rollpytorch/${{ env.PT_RELEASE }}
         git add pytorch-hash.txt pytorch-requirements.txt torchvision-requirements.txt lib/Dialect/Torch/Transforms/AbstractInterpLibrary.cpp include/torch-mlir/Dialect/Torch/IR/GeneratedTorchOps.td
-        git diff --cached --exit-code || (git commit -m "update PyTorch version to ${{ env.PT_RELEASE }}" && git push --set-upstream origin main)
+        git diff --cached --exit-code || git commit -m "update PyTorch version to ${{ env.PT_RELEASE }}"
 
-    - name: Update PyTorch Build Cache (if running on main branch)
-      if: github.ref_name == 'main'
-      id: cache-pytorch
-      uses: ashay/cache@v1
+    - name: Create pull request
+      uses: peter-evans/create-pull-request@v5.0.1
       with:
-        path: ${{ github.workspace }}/build_tools/python_deploy/wheelhouse
-        key: ${{ runner.os }}-pytorch-${{ env.PT_HASH }}
+        author: Roll PyTorch Action <torch-mlir@users.noreply.github.com>
+        branch: rollpytorch/${{ env.PT_RELEASE }}
+        body: |
+          torch version: ${{ env.PT_RELEASE }}
+          torch commit hash: ${{ env.PT_HASH }}
+          torchvision version: ${{ env.PTVISION_RELEASE }}
+        commit-message: |
+          update PyTorch version to ${{ env.PT_RELEASE }}
+
+           - torch version: ${{ env.PT_RELEASE }}
+           - torch commit hash: ${{ env.PT_HASH }}
+           - torchvision version: ${{ env.PTVISION_RELEASE }}
+        committer: Roll PyTorch Action <torch-mlir@users.noreply.github.com>
+        title: update PyTorch version to ${{ env.PT_RELEASE }}
+        reviewers: ashay, powderluv, vivekkhandelwal1