Python: Optional dependencies (#820)

### Motivation and Context
The release of Python SK from 0.2.4 -> 0.2.6 significantly increased the
size of the pip package. The dependencies causing this are not core to
the kernel, so this PR establishes a pattern for SK connector
dependencies (ex: Hugging Face, memory storage)

### Description
- Added a poetry group called hugging_face - this is the same name as
the connectors namespace in the code.
- Added dependency installs to the Hugging Face sample notebook.
- removed Hugging Face dependencies from requirements.txt.
- Added import errors for torch, transformers, and sentence-transformers
in the hugging_face connector classes.
- Added command `--with hugging_face` to integration test setup.

.github/workflows/python-integration-tests.yml

@@ -36,7 +36,7 @@ jobs:
       run: |
         python -m pip install --upgrade pip
         python -m pip install poetry pytest
-        cd python && poetry install
+        cd python && poetry install --with hugging_face
     - name: Run Integration Tests
       shell: bash
       env: # Set Azure credentials secret as an input