add interface to locally installed Alpaca LLM (#2297)
blatantly copying the code proposal from the issue
https://github.com/hwchase17/langchain/issues/1777 written by
https://gist.github.com/lukestanley
```
from langchain.llms import alpaca
llm=alpaca.Llama()
llm("What are sights to visit in Rome?")
```
Paths set in the defaults are the default paths from
[Dalai](https://github.com/cocktailpeanut/dalai).
