Add sample parameter to Faiss ANN

[davidmezzetti]

Currently when building a Faiss index, all rows are used in the training step. For larger indexes, using all rows isn't necessary to build an accurate ANN index. A fraction of the data works well and vastly reduces indexing time. See the link below for more on this.

https://github.com/facebookresearch/faiss/wiki/FAQ#how-can-i-distribute-index-building-on-several-machines

This issue will add a new parameter called sample. When set, this parameter will select a fraction of the data using random sampling. Once the model is trained, all rows will be added to the ANN index.