Allow clearing cache and fix gptcache (#3493)

This PR

* Adds `clear` method for `BaseCache` and implements it for various
caches
* Adds the default `init_func=None` and fixes gptcache integtest
* Since right now integtest is not running in CI, I've verified the
changes by running `docs/modules/models/llms/examples/llm_caching.ipynb`
(until proper e2e integtest is done in CI)

docs/modules/models/llms/examples/llm_caching.ipynb

@@ -785,7 +785,9 @@
    "id": "9df0dab8",
    "metadata": {},
    "outputs": [],
-   "source": []
+   "source": [
+    "!rm .langchain.db sqlite.db"
+   ]
   }
  ],
  "metadata": {