Add qnn backend to pip package (pytorch#13733)

Summary:

When generating the wheel packages
- download the qnn sdk such that we can build the qnn backend
- remove the downloaded sdk files so it won't be part of wheel packages

Inside the qnn packages, it includes the AoT part and the the qnn backend.so files (the build from executorch/backends/qualcomm)

For users:
```
# This step will install qnn backend
pip install executorch-0.8.0a0+e5f94da-cp310-cp310-linux_x86_64.whl


# The qnn sdk will be downloaded when users first , if the QNN_SDK_ROOT is not set. This step will only run once 
from executorch.backend.qualcomm import any module
```

Rollback Plan:

Differential Revision: D80992066

Author: cccclai

.ci/scripts/test_wheel_package_qnn.sh

@@ -0,0 +1,71 @@
+#!/bin/bash
+
+set -e  # Exit on any error
+
+echo "=== Building Wheel Package ==="
+python setup.py bdist_wheel
+
+# Find the wheel file
+WHEEL_FILE=$(ls dist/*.whl | head -n 1)
+echo "Found wheel: $WHEEL_FILE"
+
+echo "=== Checking for expected .so files ==="
+SO_FILES=$(unzip -l "$WHEEL_FILE" | grep "\.so" | grep "qualcomm")
+
+# Check for the three expected .so files
+if echo "$SO_FILES" | grep -q "executorch/backends/qualcomm/qnn_backend.cpython-310-x86_64-linux-gnu.so" && \
+   echo "$SO_FILES" | grep -q "executorch/backends/qualcomm/python/PyQnnManagerAdaptor.cpython-310-x86_64-linux-gnu.so" && \
+   echo "$SO_FILES" | grep -q "executorch/backends/qualcomm/python/PyQnnWrapperAdaptor.cpython-310-x86_64-linux-gnu.so"; then
+    echo "All expected .so files found in wheel"
+else
+    echo "ERROR: Missing expected .so files"
+    exit 1
+fi
+
+# Create a temporary directory for our test environment
+TEMP_ENV_DIR=$(mktemp -d)
+echo "Using temporary directory: $TEMP_ENV_DIR"
+
+echo "=== Creating and testing in conda environment ==="
+conda create -y -p "$TEMP_ENV_DIR/env" python=3.10
+conda run -p "$TEMP_ENV_DIR/env" pip install "$WHEEL_FILE"
+
+echo "=== Testing import without SDK download ==="
+conda run -p "$TEMP_ENV_DIR/env" python -c "import executorch; print('executorch imported successfully')"
+
+# Check that SDK directory doesn't exist after first import
+SDK_PATH="$TEMP_ENV_DIR/env/lib/python3.10/site-packages/executorch/backends/qualcomm/sdk"
+if [ -d "$SDK_PATH" ]; then
+    echo "ERROR: SDK directory exists after first import: $SDK_PATH"
+    exit 1
+else
+    echo "SDK directory correctly doesn't exist after first import"
+fi
+
+echo "=== Testing import that should trigger SDK download ==="
+conda run -p "$TEMP_ENV_DIR/env" python -c "import executorch.backends.qualcomm; print('executorch.backends.qualcomm imported successfully')"
+
+# Check that SDK directory exists after second import
+if [ -d "$SDK_PATH" ]; then
+    echo "SDK directory correctly exists after second import: $SDK_PATH"
+else
+    echo "ERROR: SDK directory doesn't exist after second import"
+    exit 1
+fi
+
+echo "=== Running model generation script ==="
+conda run -p "$TEMP_ENV_DIR/env" python script.py
+
+# Check if linear.pte file was created
+if [ -f "linear.pte" ]; then
+    echo "Model file linear.pte successfully created"
+else
+    echo "ERROR: Model file linear.pte was not created"
+    exit 1
+fi
+
+echo "=== Cleaning up ==="
+conda env remove -p "$TEMP_ENV_DIR/env" -y
+rm -rf "$TEMP_ENV_DIR"
+
+echo "=== All tests passed! ==="