[DOCS] Fix in example - Using a DummyLLM to avoid loading one

[kgdrathan]

Which page or section is this issue related to?

url: https://distilabel.argilla.io/latest/sections/how_to_guides/basic/task/
source: https://github.com/argilla-io/distilabel/blob/main/docs/sections/how_to_guides/basic/task/index.md

What are you documenting, or what change are you making in the documentation?

1. DummyLLM is of type AsyncLLM and generate method has been defined instead of agenerate - Changed to LLM and kept generate method as is.
2. MagpieChatTemplateMixin is not imported in __init__.py of mixins module. Change has been made to import directly from the magpie.py file in mixins module.