Loading a faiss index KeyError
Hi !

The code of the example is valid.
An index is a search engine, it's not considered a column of a dataset.
When you do `ds.load_faiss_index("embeddings", 'my_index.faiss')`, it attaches an index named "embeddings" to the dataset but it doesn't re-add the "embeddings" column. You can list the indexes of a dataset by using `ds.list_indexes()`.

If I understand correctly by reading this example you thought that it was re-adding the "embeddings" column.
This looks misleading indeed, and we should add a note to make it more explicit that it doesn't store the column that was used to build the index.

Feel free to open a PR to suggest an improvement on the documentation if you want to contribute :)
I've recently been testing out RAG and DPR embeddings, and I've run into an issue that is not apparent in the documentation.

The basic steps are:

1. Create a dataset (dataset1)
2. Create an embeddings column using DPR
3. Add a faiss index to the dataset
4. Save faiss index to a file
5. Create a new dataset (dataset2) with the same text and label information as dataset1
6. Try to load the faiss index from file to dataset2
7. Get `KeyError: "Column embeddings not in the dataset"`

I've made a colab notebook that should show exactly what I did. Please switch to GPU runtime; I didn't check on CPU.

https://colab.research.google.com/drive/1X0S9ZuZ8k0ybcoei4w7so6dS_WrABmIx?usp=sharing

Ubuntu Version
VERSION="18.04.5 LTS (Bionic Beaver)"

datasets==1.4.1
faiss==1.5.3
faiss-gpu==1.7.0
torch==1.8.0+cu101
transformers==4.3.3

NVIDIA-SMI 460.56
Driver Version: 460.32.03
CUDA Version: 11.2    
Tesla K80           

I was basically following the steps here: https://huggingface.co/docs/datasets/faiss_and_ea.html#adding-a-faiss-index

I included the exact code from the documentation at the end of the notebook to show that they don't work either.

Loading a faiss index KeyError 
 I've recently been testing out RAG and DPR embeddings, and I've run into an issue that is not apparent in the documentation.

The basic steps are:

1. Create a dataset (dataset1)
2. Create an embeddings column using DPR
3. Add a faiss index to the dataset
4. Save faiss index to a file
5. Create a new dataset (dataset2) with the same text and label information as dataset1
6. Try to load the faiss index from file to dataset2
7. Get `KeyError: "Column embeddings not in the dataset"`

I've made a colab notebook that should show exactly what I did. Please switch to GPU runtime; I didn't check on CPU.

https://colab.research.google.com/drive/1X0S9ZuZ8k0ybcoei4w7so6dS_WrABmIx?usp=sharing

Ubuntu Version
VERSION="18.04.5 LTS (Bionic Beaver)"

datasets==1.4.1
faiss==1.5.3
faiss-gpu==1.7.0
torch==1.8.0+cu101
transformers==4.3.3

NVIDIA-SMI 460.56
Driver Version: 460.32.03
CUDA Version: 11.2    
Tesla K80           

I was basically following the steps here: https://huggingface.co/docs/datasets/faiss_and_ea.html#adding-a-faiss-index

I included the exact code from the documentation at the end of the notebook to show that they don't work either.
 
 Hi !

The code of the example is valid.
An index is a search engine, it's not considered a column of a dataset.
When you do `ds.load_faiss_index("embeddings", 'my_index.faiss')`, it attaches an index named "embeddings" to the dataset but it doesn't re-add the "embeddings" column. You can list the indexes of a dataset by using `ds.list_indexes()`.

If I understand correctly by reading this example you thought that it was re-adding the "embeddings" column.
This looks misleading indeed, and we should add a note to make it more explicit that it doesn't store the column that was used to build the index.

Feel free to open a PR to suggest an improvement on the documentation if you want to contribute :)