Add tests for imports when optional packages are unavailable

.github/workflows/ci_test-full.yml

@@ -66,6 +66,11 @@ jobs:
  python --version
  pip --version
  # python -m pip install --upgrade --user pip
+
+ # Install only mandatory dependencies and check if `import pl_bolts` is successful without any optional packages.
+ pip install --requirement requirements.txt --upgrade --quiet --find-links https://download.pytorch.org/whl/cpu/torch_stable.html
+ python -c 'import pl_bolts'
+
  pip install --requirement requirements/devel.txt --upgrade --quiet --find-links https://download.pytorch.org/whl/cpu/torch_stable.html
  pip list
  shell: bash