[onnx] Add ONNX importer and iree-import-onnx tool to compiler package. (#15920)

* When building the torch frontend, we now also have access to the
upstream ONNX importer and include it here as part of our official API.
* Also added a custom `iree-import-onnx` tool and corresponding test.
* Added extras_require for `onnx` to setup.py (allowing it to be
installed as an optional dependency).
* Added a _package_test.py for the compiler package like the runtime has
and configured the CI to use it.
* Added a check to the release validation job.
* Includes a bump of torch-mlir to latest.
* May need to tweak some things in the input pipeline to get
iree-compile to work on this by default. Will do in a followup.
diff --git a/.github/workflows/validate_and_publish_release.yml b/.github/workflows/validate_and_publish_release.yml
index 41d19b4..0d8d85a 100644
--- a/.github/workflows/validate_and_publish_release.yml
+++ b/.github/workflows/validate_and_publish_release.yml
@@ -41,7 +41,12 @@
       - name: Install python packages
         id: install_python_packages
         run: |
-          python -m pip install -f file://$PWD/artifact/ iree-compiler iree-runtime iree-tools-tflite iree-tools-tf
+          python -m pip install -f file://$PWD/artifact/ iree-compiler[onnx] iree-runtime iree-tools-tflite iree-tools-tf
+      - name: Validate IREE Compiler Package
+        id: validate_compiler_package
+        run: |
+          echo "Testing compiler package:"
+          python -m iree.compiler._package_test
       - name: Validate IREE Runtime Package
         id: validate_runtime_package
         run: |