docs, tests: replace datasets.rst with memory_optimizations.rst (#968)

datasets.rst was removed from torchtune repo.

Signed-off-by: Ihar Hrachyshka <ihar.hrachyshka@gmail.com>

# What does this PR do?

Replace a missing 404 document with another one that exists. (Removed it
from
the list when memory_optimizations.rst was already pulled.)


## Test Plan

Please describe:
 - tests you ran to verify your changes with result summaries.
 - provide instructions so it can be reproduced.


## Sources

Please link relevant resources if necessary.


## Before submitting

- [x] This PR fixes a typo or improves the docs (you can dismiss the
other checks if that's the case).
- [x] Ran pre-commit to handle lint / formatting issues.
- [ ] Read the [contributor
guideline](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [ ] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

Signed-off-by: Ihar Hrachyshka <ihar.hrachyshka@gmail.com>

docs/getting_started.ipynb

@@ -1662,7 +1662,7 @@
         "from termcolor import cprint\n",
         "from llama_stack_client.types import Document\n",
         "\n",
-        "urls = [\"chat.rst\", \"llama3.rst\", \"datasets.rst\", \"lora_finetune.rst\"]\n",
+        "urls = [\"chat.rst\", \"llama3.rst\", \"memory_optimizations.rst\", \"lora_finetune.rst\"]\n",
         "documents = [\n",
         "    Document(\n",
         "        document_id=f\"num-{i}\",\n",